Article

Electronic Structure and Chemical Bonding in Ti2AlC Investigated by Soft X-ray Emission Spectroscopy

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Abstract

The electronic structure of the nanolaminated transition metal carbide Ti2AlC has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Ti L, C K, and Al L emission spectra are compared with calculated spectra using ab initio density-functional theory including dipole matrix elements. The detailed investigation of the electronic structure and chemical bonding provides increased understanding of the physical properties of this type of nanolaminates. Three different types of bond regions are identified: The relatively weak Ti 3d–Al 3p bond 1 eV below the Fermi level and the Ti 3d–C 2p and Ti 3d–C 2s bonds which are stronger and deeper in energy are observed around 2.5 and 10 eV below the Fermi level, respectively. A strongly modified spectral shape of the 3s final states in comparison to pure Al is detected for the intercalated Al monolayers indirectly reflecting the Ti 3d–Al 3p hybridization. The differences between the electronic and crystal structures of Ti2AlC, Ti3AlC2, and TiC are discussed in relation to the number of Al layers per Ti layer in the two former systems and the corresponding change of the unusual materials properties.

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... The important family of ternary carbide and nitride compounds, so-called M n+1 AX nphases is the subject of intense research [1]. Three different kinds of crystal structures (stoichiometries) are classified as 211 (n=1), 312 (n=2) and 413 (n=3) phases [2][3][4]. Here, the letter M denotes an early transition metal, A is an element in the groups III-V and X is either carbon or nitrogen. The MAX-phases exhibit a technologically important combination of metallic and ceramic properties, including high strength and toughness at high temperature, resistance to oxidation and thermal shock, exhibit high electrical and thermal conductivity [1], is environmentally friendly and relatively cheap to produce. ...
... The x-ray photons were detected parallel to the polarization vector of the incoming beam in order to minimize elastic scattering. The deposition procedure of the epitaxially grown thin film coatings are described elsewhere [4,11]. Figure 1 shows soft x-ray emission spectra of Ti 2 AlC (full curves) and TiC (dotted curves) excited nonresonantly above the Ti L (top), C K (middle) and Al L edges (bottom). ...
... The excitation energies were 477 eV, 310 eV and 120 eV, respectively. For comparison, the spectra are normalized to unity and plotted on a common energy scale relative to the Fermi level (E F ) using the core level XPS binding energies of Ti 2 AlC [4]. For Ti 2p 1/2 , C 1s and Al 2p 1/2 , 460.3 eV, 281.9 eV and 72 eV binding energies were used, respectively. ...
The electronic structure of nanolaminate Ti2AlC and Ti2AlN thin films, so-called MAX-phases, were investigated by soft X-ray emission spectroscopy. These nanolaminated carbide and nitride compounds represent a class of layered materials with a combination of properties from both metals and ceramics. The bulk-sensitive soft X-ray emission technique is particularly useful for detecting detailed electronic structure information about internal monolayers and interfaces. The Ti-Al bonding is manifested by a pronounced peak in the Ti L-emission of Ti2AlC and Ti2AlN that is not present in the binary TiC system. The spectral shape of Al L-emission in the MAX-phase is strongly modified in comparison to metallic Al. By replacing or partly exchanging C with N, a change of the electron population can be achieved causing a change of covalent bonding between the laminated layers, which enables control of the material properties.
... The low intensity of the impurity peaks compared to the Ti 3 AC 2 (A = Al, Si, Ge) phase peaks is due to the fact that these impurity concentrations are very small and that their contributions to X-ray spectroscopy measurements can be disregarded. Similar diffractograms were also found for other thin film MAX phases [16,[24][25][26][27]. ...
... Thus, the covalent Ti 3d\ \N 2p bonding in TiN is significantly stronger than the Ti 3d\ \C 2p bonding in TiC. By further analyzing the partial DOS for Ti 2 AlC and Ti 2 AlN (Fig. 9), their crystal overlap population data (Section 7) and bond lengths (Section 6), the same difference as in the binaries is observed for Ti 2 AlC [27] and Ti 2 AlN [24]. This general difference tendency is also confirmed by the energy shift in the experimental X-ray emission spectra of Ti 2 AlC in comparison to Ti 2 AlN (Fig. 8). ...
... A complicating factor may be the strong electron-phonon coupling with a Ge oscillation along the c-axis that has a higher frequency than 13. Computed electron density difference plot along the [110] plane between Ti 2 AlC and Ti 2 C 2 (TiC) in the same crystal geometry [27]. The difference density plot was obtained by subtracting the charge densities in the 110 diagonal plane of the hexagonal unit cell. ...
Article
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The low intensity of the impurity peaks compared to the Ti 3 AC 2 (A = Al, Si, Ge) phase peaks is due to the fact that these impurity concentrations are very small and that their contributions to X-ray spectroscopy measurements can be disregarded. Similar diffractograms were also found for other thin film MAX phases [16,[24][25][26][27]. ...
... Thus, the covalent Ti 3d\ \N 2p bonding in TiN is significantly stronger than the Ti 3d\ \C 2p bonding in TiC. By further analyzing the partial DOS for Ti 2 AlC and Ti 2 AlN (Fig. 9), their crystal overlap population data (Section 7) and bond lengths (Section 6), the same difference as in the binaries is observed for Ti 2 AlC [27] and Ti 2 AlN [24]. This general difference tendency is also confirmed by the energy shift in the experimental X-ray emission spectra of Ti 2 AlC in comparison to Ti 2 AlN (Fig. 8). ...
... A complicating factor may be the strong electron-phonon coupling with a Ge oscillation along the c-axis that has a higher frequency than 13. Computed electron density difference plot along the [110] plane between Ti 2 AlC and Ti 2 C 2 (TiC) in the same crystal geometry [27]. The difference density plot was obtained by subtracting the charge densities in the 110 diagonal plane of the hexagonal unit cell. ...
Article
Full-text available
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The main peaks were explained in terms of core-shell spin-orbit splitting of 13.2 eV, 33 and their each double-subpeaks shown in Fig. 1(f) has been observed before in transitional-metal carbides and they correspond to the usual ligand-field splitting (≈2.0 eV). [34][35][36] Figure 2 shows the normalized C K-edge and O K-edge X-ray absorption near edge structure (XANES) spectra of GO, r-GO, and r-GO-ATA-Fe 2 O 3 nanocomposites. The spectrum in Fig. 2 clearly shows the presence of states, unoccupied π* (1s → π*) at 2.85.2 eV and σ* (1s → σ*) at 291.0 eV, indicating that GO/r-GO/r-GO-ATA-Fe 2 O 3 nanocomposites still display the aromaticity of the original pristine graphene. ...
... These four peaks are assigned to sp 2 CvC, hydroxyl/phenolic group, epoxy group, and >CvO, respectively. 28,36,40,41 In the case of r-GO, these peaks are shifted toward lower energy at ∼281.5 eV ( pre-sp 2 -C), ∼283.2 eV (sp 2 -C), ∼284.2 eV (sp 2 -C), and ∼285.5 eV (CZO), respectively. 28,36,40,41 In the case of the r-GO-ATA-Fe 2 O 3 nanocomposite, the bond structure, FeZC at ∼282.8 eV (peak-I), CZFe and/or sp 2 -C ∼532 eV is associated with CZO phenolic group (oxygen singly bonded to aliphatic carbon). ...
... 28,36,40,41 In the case of r-GO, these peaks are shifted toward lower energy at ∼281.5 eV ( pre-sp 2 -C), ∼283.2 eV (sp 2 -C), ∼284.2 eV (sp 2 -C), and ∼285.5 eV (CZO), respectively. 28,36,40,41 In the case of the r-GO-ATA-Fe 2 O 3 nanocomposite, the bond structure, FeZC at ∼282.8 eV (peak-I), CZFe and/or sp 2 -C ∼532 eV is associated with CZO phenolic group (oxygen singly bonded to aliphatic carbon). The peak at ∼527.6 (±0.1) eV is associated with carbon -oxygen and the peak at 528.7 eV is attributed to oxides of iron. ...
Article
We have synthesized r-GO-ATA-Fe2O3 nanocomposites and studied their microstructural and electromagnetic properties for future possible magnetic resonance imaging for biomedical application. X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption near edge spectroscopy were used to study the structural and electronic properties, while a superconducting quantum interface device magnetometer was used for investigating the magnetic behavior of the nanocomposites. The nanocomposites have been found to reduce the graphitic structure of GO due to the substitution of carbon/oxygen and/or iron nanoparticles. Conversely, the electrical conductivity of nanocomposites is found to be high due to the formation of Fe—C/Fe—O bonds in the structure of the nanocomposites. The composites also exhibit superparamagnetic features as observed from the M-H hysteresis loop with saturation magnetization of ≈0.1 emu/g at 1.8 K temperature. The results, in general, suggest possible applicability of r-GO/Fe2O3 nanocomposites as an effective multifunctional platform for magnetic resonance imaging in biomedical applications.
... Literature review shows that Ti (2p) signals are positioned at [20,41] and bulk [42,43] and commercial Maxthal 211 bulk ceramic (nominally Ti 2 AlC, Kanthal/Sweden) [44]. Ti-O x bonds around 459 eV and 464.58 eV are related to the adsorption and reaction of oxygen with the outer layers of the coating based on the previous detection of these peaks in the cold spraying of Ti 2 AlC MAXphase coatings [45]. ...
... C (1 s) spectrum shows a peak at 281.8 eV for Ti 3 AlC 2 and Ti 2 AlC films, which originates from carbon in carbide state (Ti-C bond) [20,41,42]. The peaks located at 284 eV is related to amorphous matrix of non-carbide carbon [20,42], while 285.3 and 289.3 eV are related to the hydrocarbon (-CH 2 -& -CH 3 ) and carboxylate (-COO) contamination common for samples exposed to laboratory air [41] and also 286.6 eV is related to hydroxyl (-C-OH) bonds [47]. ...
... C (1 s) spectrum shows a peak at 281.8 eV for Ti 3 AlC 2 and Ti 2 AlC films, which originates from carbon in carbide state (Ti-C bond) [20,41,42]. The peaks located at 284 eV is related to amorphous matrix of non-carbide carbon [20,42], while 285.3 and 289.3 eV are related to the hydrocarbon (-CH 2 -& -CH 3 ) and carboxylate (-COO) contamination common for samples exposed to laboratory air [41] and also 286.6 eV is related to hydroxyl (-C-OH) bonds [47]. Thus the peaks located at~285, 287 and 289 eV in Fig. 5c are likely to be related to C-X bonds (X = C, O, H) at the coating's surface and 281.9 eV is to be related to Ti-C bonds. ...
Article
MAX phases are appropriate protective coatings due to their unique properties such as excellent corrosion resistance. In this study, Ti-Al-C coatings with different percentages of Ti, Al and C were deposited on stainless steel and quartz glass substrates by Arc-PVD using two different Ti-Al alloy targets with (1:1) and (1:2) atomic ratios in C2H2/Ar atmosphere with various ratios (1/10, 1/4, 1/2). Then, the coated substrates were annealed in a vacuum furnace at different temperatures between 600 and 1000 °C. The results of XRD analysis indicated that Ti2AlC MAX phase is generated from 1Ti-2Al target, while Ti2AlC and Ti3AlC2 MAX phases are produced from 1Ti-1Al target by annealing at temperatures above 800 °C. Based on the results of this study, the best C2H2/Ar ratio in the Arc-PVD atmosphere for the synthesis of MAX phases was 1/4. Raman spectroscopy and XPS confirmed the presence of the MAX phases in the coatings. EDS analysis showed that the overall compositions occur within the regions in the ternary phase diagram in which the MAX phases are stable. Keywords: MAX phases; Ti2AlC; Ti3AlC2; Ti-Al-C; Coating; Arc-PVD; Annealing; Vacuum.
... Each subspectrum further split in double sub-peak due to ligand field splitting marked as II and IV in Fig. 4. Such splitting has been observed in several transition metal-carbides [34,[47][48][49]. The spectra show two characteristic change with T s (i) peak become narrow, and (ii) the total integrated intensity of the feature is decreasing as shown in the inset of Fig. 4. ...
... In addition, the shoulder a can be solely due to formation of metal carbide [34,[47][48][49]. The intensity of this shoulder is faint and does not show any significant change. ...
Article
We studied the structural and magnetic properties of Fe0.8C0.2 thin films deposited by co-sputtering of Fe and C targets in a direct current magnetron sputtering (dcMS) process at a substrate temperature (Ts) of 300, 523, and 773 K. The structure and morphology were measured using x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES) at Fe L and C K edges and atomic/magnetic force microscopy (AFM, MFM). An ultrathin (3-nm) Fe0.857C0.2 layer, placed between relatively thick Fe0.8C0.2 layers was used to estimate Fe self-diffusion taking place during growth at different Ts using depth profiling measurements. Such Fe0.857C0.2 layer was also used for Fe57 conversion electron Mössbauer spectroscopy (CEMS) and nuclear resonance scattering (NRS) measurements, yielding the magnetic structure of this ultrathin layer. We found from XRD measurements that the structure formed at low Ts (300 K) is analogous to Fe-based amorphous alloy and at high Ts (773 K), predominantly a Fe3C phase has been formed. Interestingly, at an intermediate Ts (523 K), a clear presence of Fe4C (along with Fe3C and Fe) can be seen from the NRS spectra. The microstructure obtained from AFM images was found to be in agreement with XRD results. MFM results also agree well with NRS as the presence of multi-magnetic components can be clearly seen in the sample grown at Ts = 523 K. The information about the hybridization between Fe and C, obtained from Fe L- and C K-edge XANES also supports the results obtained from other measurements. In essence, from this work, a possibility for experimental realization of Fe4C has been demonstrated. It can be anticipated that by further fine-tuning of the deposition conditions, even single-phase Fe4C can be realized which hitherto remains an experimental challenge.
... The ternary phases Ti 2 AlC andTi 3 AlC 2 belong to a family of ternary carbides with a general formulaM n+1 AX n (MAX), where, M is an early transition metal, A is an A-group element (mostly groups 13 and 14) and X is C or N [1][2][3]. The MAX phases combine some of the best attributes of metals and ceramics. ...
... Between 300 and 800 K, ΔG of Eq. (3) > 0,indicating that T 2 AlC did not decompose, however, when the temperature is higher than 800 K, ΔG (3) < 0, showing that Eq. (3) can happen above 800 K. When the TAC comes into contact with the Ti filler-material during TIG brazing process, it starts decomposing according to Eqs. (1), (2) and/or (3). The spreading of the molten Ti is followed by a deep penetration of the Ti liquid into the grain boundaries of the substrateup to 280 µm approximately from the TAC/Ti interface (Fig. 4a). ...
Article
Herein we study the infiltration behavior of Ti and Cu fillers into a Ti2AlC/Ti3AlC2MAX phase composites using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti2AlC/Ti3AlC2 comes into contact with molten Ti, it starts decomposing into TiCx, a Ti-richandTi3AlC; when in contact with molten Cu, the resulting phases are Ti2Al(Cu)C, Cu(Al), AlCu2Ti and TiC. In the presence of Cu at approximately 1630°C, a defective Ti2Al(Cu)C phase was formed having a P63/mmc structure. Ti3AlC2 MAX phase was completely decomposed in presence of Cu or Ti filler-materials. The decomposition of Ti2AlC to Ti3AlC2 was observed in the heat-affected zone of the composite. Notably, no cracks were observed during TIG-brazing of Ti2AlC/Ti3AlC2 composite with Ti or Cu filler materials.
... On the basis of density-functional theory (DFT) and ab initio calculations of the electronic structure [50,60,[72][73][74][75][76][77][78][79][80][81][82][83][84][85][86], as well as relying on the data of novel computation methods, such as the full-potential band-structure method (FP-LAPW) [87,88], pseudopotential plane-wave method (PP-PW) [89], DFT + U [90] and the hybrid functionals [91,92], M 2 AlC phases can be classified into two groups: weakly coupled (M = Sc, Ti, Zr, Hf) and strongly coupled (M = V, Nb, Ta, Cr, Mo, W) nanolaminates, in accordance with the valence electron concentration (VEC) of the transition metal M. There exists common agreement in the literature about the general tendency that the transition metal atoms, more specifically their 3d and 4s orbitals, are the main charge reservoir which always lose electronic charge, while C-, N-or A-element tend to gain charge, upon bond formation. ...
... For this reason, Cui et al. predicted Cr 2 AlC to be more brittle than Cr 2 AlN [94]. X-ray spectroscopic studies have confirmed this tendency in the general difference of the chemical bonding schemes of MAX phase nitrides and carbides [72,[78][79][80][81][82]. ...
Article
Part III of this overview presents a summary of the potential multifunctional applications of MAX phase materials. Coatings of these materials have been investigated for a range of uses, such as for: high-temperature electrical contacts, microelectronic layers, magnetic and optical materials, fuel holder protection in the nuclear industry, oxidation, corrosion and erosion protection, bio-compatible material, thermal barriers, protective aerospace coatings and as armour in defence applications. What makes this material useful for many of these applications is its excellent mechanical properties, damage tolerance, self-healing, high-temperature melting point, and its outstanding oxidation, corrosion and abrasion resistance. Particular attention is given to an aircraft engine's design and the related materials challenges. A comparison is made to the currently utilized turbine surface coatings, as well as the motivation behind the usage of these new high performance MAX phase coatings, the nature of their protection/wear and other strengths/weaknesses.
... The Raman sp 2 -fraction was estimated from [23]. [7,25,36]. A comparison of the spectral shapes at different carbon contents shows two interesting effects: (i) the total intensity of the spectra increases with carbon content as in the case of Cr 2p XAS in amorphous CrC x [7], and Ti 2p XAS in nc-TiC/a-C nanocomposites that exhibit the same trend [5]. ...
... As determined from the XPS results shown in figures 3 and 4, the C-C and C-Fe binding energies are essentially the same for all compositions. Generally, the 1.7 eV difference in binding energy of the C-Fe peak in comparison to the C-C peak signifies electronic charge-transfer from Fe to C. This chemical shift in XPS is smaller than in the case of Ti to C [8,36] due to the smaller difference in electronegativity for Fe and C. ...
Article
We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe 1−x C x (0.21 x 0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe 1−x C x films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeC y , and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe 3 C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From x-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15–25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit an increasing number of unoccupied 3d states and a decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size-dependent carbide/matrix interfaces. The four-point probe resistivity of the Fe 1−x C x films increases exponentially with carbon content from ∼200 µµ cm (x = 0.21) to ∼1200 µµcm (x = 0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides through the control of amorphous domain structures.
... Each subspectrum further split in double sub-peak due to ligand field splitting marked as II and IV in Fig. 4. Such splitting has been observed in several transition metal-carbides [34,[47][48][49]. The spectra show two characteristic change with T s (i) peak become narrow, and (ii) the total integrated intensity of the feature is decreasing as shown in the inset of Fig. 4. ...
... In addition, the shoulder a can be solely due to formation of metal carbide [34,[47][48][49]. The intensity of this shoulder is faint and does not show any significant change. ...
Preprint
We studied the structural and magnetic properties of \FeC~thin films deposited by co-sputtering of Fe and C targets in a direct current magnetron sputtering (dcMS) process at a substrate temperature (\Ts) of 300, 523 and 773\,K. The structure and morphology was measured using x-ray diffraction (XRD), x-ray absorption near edge spectroscopy (XANES) at Fe $L$ and C $K$-edges and atomic/magnetic force microscopy (AFM, MFM), respectively. An ultrathin (3\,nm) $^{57}$\FeC~layer, placed between relatively thick \FeC~layers was used to estimate Fe self-diffusion taking place during growth at different \Ts~using depth profiling measurements. Such $^{57}$\FeC~layer was also used for $^{57}$Fe conversion electron M\"{o}ssbauer spectroscopy (CEMS) and nuclear resonance scattering (NRS) measurements, yielding the magnetic structure of this ultrathin layer. We found from XRD measurements that the structure formed at low \Ts~(300\,K) is analogous to Fe-based amorphous alloy and at high \Ts~(773\,K), pre-dominantly a \tifc~phase has been formed. Interestingly, at an intermediate \Ts~(523\,K), a clear presence of \tefc~(along with \tifc~and Fe) can be seen from the NRS spectra. The microstructure obtained from AFM images was found to be in agreement with XRD results. MFM images also agrees well with NRS results as the presence of multi-magnetic components can be clearly seen in the sample grown at \Ts~= 523\,K. The information about the hybridization between Fe and C, obtained from Fe $L$ and C $K$-edges XANES also supports the results obtained from other measurements. In essence, from this work, experimental realization of \tefc~has been demonstrated. It can be anticipated that by further fine-tuning the deposition conditions, even single phase \tefc~phase can be realized which hitherto remains an experimental challenge.
... On the basis of density-functional theory (DFT) and ab initio calculations of the electronic structure [50,60,[72][73][74][75][76][77][78][79][80][81][82][83][84][85][86], as well as relying on the data of novel computation methods, such as the full-potential band-structure method (FP-LAPW) [87,88], pseudopotential plane-wave method (PP-PW) [89], DFT + U [90] and the hybrid functionals [91,92], M 2 AlC phases can be classified into two groups: weakly coupled (M = Sc, Ti, Zr, Hf) and strongly coupled (M = V, Nb, Ta, Cr, Mo, W) nanolaminates, in accordance with the valence electron concentration (VEC) of the transition metal M. There exists common agreement in the literature about the general tendency that the transition metal atoms, more specifically their 3d and 4s orbitals, are the main charge reservoir which always lose electronic charge, while C-, N-or A-element tend to gain charge, upon bond formation. ...
... For this reason, Cui et al. predicted Cr 2 AlC to be more brittle than Cr 2 AlN [94]. X-ray spectroscopic studies have confirmed this tendency in the general difference of the chemical bonding schemes of MAX phase nitrides and carbides [72,[78][79][80][81][82]. ...
Article
Recent research has highlighted the potential of MAX phase properties including its machinability, good mechanical behaviour, high electric and thermal conductivity as well as good oxidation and erosion resistance to a range of applications including electrical contact coatings, high-temperature heating elements, and barrier- and protection coatings in gas-turbine engine. Successful engineering of MAX phases requires an understanding of how the manufacturing conditions and the deposition parameters influence the composition, crystallographic, electronic structure and morphology. It is, in turn, important to know how these determine the anisotropy of physical, mechanical and technological properties of the material. A focus point of this part of the review is the systemic study of the regular trends in the preparation parameter–structure–properties relationships of MAX phase coatings. In particular, the Cr–Al–C (Y) coating synthesised by the HIPIMS technique will be considered and compared with other synthesised MAX phase bulk ceramics and coatings.
... MAX phases present a hexagonal layered crystal structure and bridge the gap between metallic and ceramic properties [19][20][21][22][23]. Comparing with Al 2 O 3 , TiB 2 , and ZrB 2 , Ti 3 AlC 2 presents high elastic moduli, high electrical conductivity, and low density relatively [24][25][26][27], which make Ti 3 AlC 2 a potentially attractive dispersion phase for Cu matrix composites. For example, Cu composite with 60 vol% ex situ Ti 3 AlC 2 particles prepared by ball milling for 10 h followed by hot press exhibited a superior ultimate compressive strength (1242 ± 24 MPa) and low electrical resistance (0.32 × 10 −6 Ω·m) [19]. ...
... A large number of dimples with uniform sizes distributed on the fracture surface of the Gd 2 O 3 /Cu composites are shown in Figure 10a, and these typical ductile fractures indicated that the Gd 2 O 3 /Cu composite was plastically deformed before fracture. The enlarged image in Figure 10b showed in situ Gd 2 O 3 particles existing at the bottom of the dimples, which proved the movement of the dislocation would be impeded by the in situ Gd 2 O 3 particles during the tensile test and helped to improve the tensile strength of the Gd 2 O 3 /Cu composite [26,27]. The size of the in situ Gd 2 O 3 particles was about 20 nm, and the value was consistent with the results in Figure 6a,b and Figure 8c. ...
Article
Full-text available
Ti3AlC2 presents a hexagonal layered crystal structure and bridges the gap between metallic and ceramic properties, and Gadolinia (Gd2O3) has excellent thermodynamic stability, which make them potentially attractive as dispersive phases for Cu matrix composites. In this paper, Cu@Ti3AlC2-Gd2O3/Cu composites, Ti3AlC2-Gd2O3/Cu composites, and Gd2O3/Cu composites were prepared by electroless Cu plating, internal oxidation, and vacuum hot press sintering. The microstructure and the effect of the Cu plating on the properties of the Cu@Ti3AlC2-Gd2O3/Cu composites were discussed. The results showed that a Cu plating with a thickness of about 0.67 μm was successfully plated onto the surface of Ti3AlC2 particles. The ex situ Ti3AlC2 particles were distributed at the Cu grain boundary, while the in situ Gd2O3 particles with a grain size of 20 nm were dispersed in the Cu grains. The electroless Cu plating onto the surface of the Ti3AlC2 particles effectively reduces their surfactivity and improves the surface contacting state between the Cu@Ti3AlC2 particles and the Cu matrix, and reduces electron scattering, so that the tensile strength reached 378.9 MPa, meanwhile, the electrical conductivity and elongation of the Cu matrix composites was maintained at 93.6 IACS% and 17.6%.
... vicinity of the Fermi level (E F ). These states dominate the density of states (DOS) at E F and contribute to the electrical conductivity [2][3][4][5][6][7] . The anisotropy of the electronic structure leads to an anisotropic conductivity: higher conductivity within the a-b basal plane and lower conductivity along the c-axis. ...
... A canted AFM structure means that the magnetic moments of the Mn 2 C slabs across the Ga layers are not collinear and the angle between AFM coupled moments is below 180° and is schematically A magnetic spin configuration and respective spin alignment between magnetic sublattices is collinear [11][12][13] . (II) At T < T t = 214 K the Mn 2 GaC undergoes the first order phase transition, which is characterized by c-axis lattice compression (c II < c I ) and magnetic spin transformation to a non-collinear AFM [0001] 4 A state. (III) at T > T t an external magnetic field causes the parallel spin alignment and compression of the lattice along the c-axis (c III < c I ) delivering large negative magnetostriction of −450 ppm. ...
Article
Full-text available
In 2013, a new class of inherently nanolaminated magnetic materials, the so called magnetic MAX phases, was discovered. Following predictive material stability calculations, the hexagonal Mn2GaC compound was synthesized as hetero-epitaxial films containing Mn as the exclusive M-element. Recent theoretical and experimental studies suggested a high magnetic ordering temperature and non-collinear antiferromagnetic (AFM) spin states as a result of competitive ferromagnetic and antiferromagnetic exchange interactions. In order to assess the potential for practical applications of Mn2GaC, we have studied the temperature-dependent magnetization, and the magnetoresistive, magnetostrictive as well as magnetocaloric properties of the compound. The material exhibits two magnetic phase transitions. The Néel temperature is TN ~ 507 K, at which the system changes from a collinear AFM state to the paramagnetic state. At Tt = 214 K the material undergoes a first order magnetic phase transition from AFM at higher temperature to a non-collinear AFM spin structure. Both states show large uniaxial c-axis magnetostriction of 450 ppm. Remarkably, the magnetostriction changes sign, being compressive (negative) above Tt and tensile (positive) below the Tt. The sign change of the magnetostriction is accompanied by a sign change in the magnetoresistance indicating a coupling among the spin, lattice and electrical transport properties.
... In the latter case, the experiments can be performed either in emission [x-ray emission spectroscopy (XES)] or in absorption [x-ray absorption spectroscopy (XAS)]. XES and XAS, combined with first-principles calculations, were used to study the electronic structure of Ti 2 AlC [29], Ti 2 AlN [30], Ti 4 SiC 3 [31], Ti 3 AlC 2 , Ti 3 SiC 2 , and Ti 3 GeC 2 [32]. However, studies specifically dedicated to electronic structure anisotropy and polarization-dependent core-level spectroscopies have been limited to V 2 GeC [33], Ti 3 SiC 2 [34], and Ti 3 AlC 2 [35]. ...
Article
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The anisotropy of the electronic structure of the MAX phase Cr2AlC has been investigated by electron-energy-loss spectroscopy (EELS) at the C K edge, and x-ray-absorption spectroscopy (XAS) at the Al K, Cr L2,3, and Cr K edges. The experimental spectra were interpreted using either a multiple-scattering approach or a full-potential band-structure method. The anisotropy is found to be small around C atoms because of the rather isotropic nature of the octahedral site, and of the averaging of the empty C p states probed by EELS at the C K edge. In turn, a pronounced anisotropy of the charge distribution around Al atoms is evidenced from polarized XAS measurements performed on textured Cr2AlC sputtered thin films. From the analysis of the XAS data using the multiple-scattering feff code, it is demonstrated that the probed thin film is constituted of 70% (0001) and 30% (101¯3) grains oriented parallel to the film surface. A decomposition of the calculated spectrum in coordination shells allows for the ability to connect XAS fine structures to the Cr2AlC structure. Combining high-resolution data with up-to-date multiple-scattering calculations, it is shown that the crystalline orientations of the grains present in a probe of 100×100 μm2 can be determined from the Cr K edge. Interestingly, it is also revealed that a static disorder is involved in the studied thin films. These findings highlight that, given the overall agreement between experimental and calculated spectra, the Cr2AlC electronic structure is accurately predicted using density functional theory.
... (iv) The 6 eV feature [19, 31] above the main 2p 3/2 peak is prominent in Ni metal x = 0.0 that is associated with electron correlation Gaussian functions give the same trend as a comparison of the peak heights but yields a higher branching ratio for Ni metal than for the other samples. A lower 2p 3/2 /2p 1/2 branching ratio is an indication of higher ionicity (lower conductivity) for the highest carbon content333435. However, the 2p 3/2 /2p 1/2 branching ratio is a result of the ionicity for Ni, mainly in the fcc-NiC x and hcp-NiC y , carbide components, and not for the entire film. ...
Data
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... Within the MAX phases family, Ti 2 AlC is among the most studied material and several groups have investigated its structural, elastic, electronic, and transport properties [8][9][10][11][12]. In particular, the phonon spectra of this material have been measured by first-order Raman scattering [13,14]. ...
Article
A linear-response method to the density functional theory is used to derive lattice dynamics, transport spectral function and electron-phonon coupling (EPC) constant of Ti2AlC, a member of the very large class of nanolaminated conducting ceramics named MAX phase. By coupling ab initio calculations with the semi-classical Boltzmann transport theory for electron-phonon scattering, the experimentally observed anisotropic electrical transport properties of Ti2AlC are rationalized. Our results indicate that in Ti2AlC, because of the weak dependence of the EPC constant〖 λ〗_(tr,α) (α =xx and zz) on the crystallographic direction, the anisotropy of ρ(T) results from the anisotropy of the Fermi-surface. These conclusions, in contrast with those obtained on Ti3SiC2 (another member of the MAX phase family) using a similar approach, establish a correlation between the nanolaminated structure of the MAX phases and the origin of their transport properties anisotropy.
... Recently, MXenes-2D early transition metal carbides and carbonitrides-have been produced by the selective etching of the A layers from the MAX phases. 6 The latter is a large family of layered compounds, with the general formula M nþ1 AX n (n ¼ 1-3), where M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X represents C or N. 7 More than 70 MAX phases are known to exist 8 and they have led to numerous technical applications, 9 owing to their unique combination of both metallic and ceramic properties, 10,11 as well as their favorable mechanical, 12 thermal, 13 and chemical 14 characteristics. ...
Article
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We report on the electrical characterization of single MXene Ti3C2Tx flakes (where T is a surface termination) and demonstrate the metallic nature of their conductivities. We also show that the carrier density can be modulated by an external gate voltage. The density of free carriers is estimated to be 8 ± 3 × 1021 cm−3 while their mobility is estimated to be 0.7 ± 0.2 cm2/V s. Electrical measurements, in the presence of a magnetic field, show a small, but clearly discernable, quadratic increase in conductance at 2.5 K.
... In this paper, we investigate the anisotropy and the orbital occupation in the electronic structure of singlecrystal Cr 2 GeC (0001) thin films. By applying bulk-sensitive and element-specific soft x-ray absorption (XAS), x-ray emission spectroscopy (XES) and resonant inelastic x-ray scattering (RIXS), we characterize the unoccupied and occupied bands of the containing elements, respectively [17][18][19]. This enables exploring the orbital occupation of the electrons buried several hundred nanometers below the surface. ...
Article
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.
... Integration of peak areas by Gaussian functions give the same trend as a comparison of the peak heights but yields a higher branching ratio for Ni metal than for the other samples. A lower 2p 3/2 /2p 1/2 branching ratio is an indication of higher ionicity (lower conductivity) for the highest carbon content [33][34][35]. However, the 2p 3/2 /2p 1/2 branching ratio is a result of the ionicity for Ni, mainly in the fcc-NiC x and hcp-NiC y , carbide components, and not for the entire film. ...
Article
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The crystal structure and chemical bonding of magnetron-sputtering deposited nickel carbide Ni 1−x C x (0.05 x 0.62) thin films have been investigated by high-resolution x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, and soft x-ray absorption spectroscopy. By using x-ray as well as electron diffraction, we found carbon-containing hcp-Ni (hcp-NiC y phase), instead of the expected rhombohedral-Ni 3 C. At low carbon content (4.9 at%), the thin film consists of hcp-NiC y nanocrystallites mixed with a smaller amount of fcc-NiC x . The average grain size is about 10–20 nm. With the increase of carbon content to 16.3 at%, the film contains single-phase hcp-NiC y nanocrystallites with expanded lattice parameters. With a further increase of carbon content to 38 at%, and 62 at%, the films transform to x-ray amorphous materials with hcp-NiC y and fcc-NiC x nanodomain structures in an amorphous carbon-rich matrix. Raman spectra of carbon indicate dominant sp 2 hybridization, consistent with photoelectron spectra that show a decreasing amount of C–Ni phase with increasing carbon content. The Ni 3d–C 2p hybridization in the hexagonal structure gives rise to the salient double-peak structure in Ni 2p soft x-ray absorption spectra at 16.3 at% that changes with carbon content. We also show that the resistivity is not only governed by the amount of carbon, but increases by more than a factor of two when the samples transform from crystalline to amorphous.
... Attempts to synthesize stoichiometrically pure Ti 2 AlC or Ti 3 AlC have frequently resulted in the presence of a second Ti-Al-C phase, thus preventing the individual properties of Ti 2 AlC and Ti 3 AlC from being accurately experimentally characterized. Evidence of Ti 2 AlC in as deposited films of Ti 3 AlC have been observed using x-ray diffraction [12], and the presence of Ti 3 AlC phases in grown Ti 2 AlC has also been observed [13]. For example, experiments conducted by Hosoda et al report a study of a Ti 3 AlC-based composite, with Ti 2 AlC as a second phase [11]. ...
Article
Ti2AlC and Ti3AlC are both considered as candidate materials for structural applications in harsh environments. In this paper, we adopt density functional theory to study the electronic, elastic, acoustic and defect properties of Ti2AlC and Ti3AlC. It was found that the mechanical properties of Ti2AlC and Ti3AlC vary little; the bulk modulus at equilibrium of Ti3AlC is 10% higher than that of Ti2AlC. However, we found that their defect properties were very different. The migration barrier of an Al vacancy in Ti2AlC is 0.834 eV compared with 4.518 eV in Ti3AlC. The difference by a factor of six in the migration barriers is explained by the bond angle variance of the C-centred octahedral. Using the calculated elastic constants, the slowness surface of the acoustic waves is obtained using the Christoffel equation. Since Ti2AlC and Ti3AlC are often presented as a second Ti–Al–C phase in each other, the individual properties calculated can be used to assess their characterizations in experiments.
... However, the flow rate of CO 2 slowly increased with prolonged exposure time as shown in Fig. 3 (a). The structure of M 2 AX phases consists of M octahedral cages with the X atoms filling the octahedral sites and interleaved layers of pure group-A element, resulting in relatively weak bonding of Ti -Al and stronger bonding of Ti -C with high migration rate of aluminum in Ti 2 AlC [41]. During high-temperature oxidation, the outward diffusion of Al to form alumina scale usually leaves a thin Ti-rich and Al-depleted intermediate layer between the oxide scale and the substrate [30,42]. ...
Article
The oxidation behavior of bulk Ti2AlC ceramic in steam has been investigated in the temperature range of 1400 °C–1600 °C. The oxidation kinetics followed a sub-parabolic law at the early stage of oxidation, then transferred to a linear law beyond 18 h at 1400 °C, and obeyed a linear law during the whole exposure up to 24 h at 1500 °C. At the initial stage of oxidation at 1400 °C and 1500 °C, randomly Al2TiO5 isolated islands with large elongated grains were observed on the surface. A continuous inner α-Al2O3 layer with a thin discontinuous outer layer of Al2TiO5 formed with prolonged exposure time. A thin Al-depleted and Ti-rich layer beneath the α-Al2O3 scale was observed, indicating selective oxidation of Al to form alumina scale. Outward diffusion of Ti and C through grain boundaries of the α-Al2O3 scale during steady-state oxidation result in segregation of TiO2 at the grain boundaries of α-Al2O3 and formation of gaseous CO and CO2, respectively. The scale adhesion was reduced in steam compared to that in air due to the accumulation of stress, and generation of voids at the scale/substrate interface. The creep and mechanical disruption of the oxide scale contribute to the breakaway oxidation of Ti2AlC at 1400 °C and to the non-protective effect at 1500 °C. The sample was rapidly and completely consumed during isothermal oxidation at 1600 °C accompanied by release of heat and hydrogen. The maximum tolerant temperature of Ti2AlC in steam was approximate 1555 °C, which can be extended via a tailored pre-oxidation process.
... (40 μ cm). 11,35 The values obtained on the bulk sample are larger. This implies that defects limit the mean free path at low temperature. ...
Data
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... This configuration supports great electrical and thermal conductance raised from the presence of many free electrons inside the overlapping layers [2]. In fact, many of this group leave behind metallic elements as regards to the electric conductance [3]. Layered structure provides the resistance to thermal shock and allows to maintain high temperatures [4]. ...
... M n+1 AX n (MAX) phases are a class of unique materials that exhibit a combination of ceramic and metallic properties, and a mixture of covalent and metallic bonding [1][2][3] . Therefore, MAX phases possess the features of elastically stiff, strong, and heat-tolerant ceramics 4 , although their electrical and heat conductivities drop linearly with increasing temperature, as with a metal 5 . For the reported MAX phases, M represents an early transition metal, A is generally a metal element in group 13 or 14, while X is limited to C or N. Utilizing the significant difference in strength between the metallic M-A bonding and covalent M-X bonding, the A-layer can be selectively etched to form two-dimensional (2D) materials known as MXenes, which cannot be synthesized directly due to their thermodynamic metastability [6][7][8][9] . ...
Article
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Mn+1AXn phases are a large family of compounds that have been limited, so far, to carbides and nitrides. Here we report the prediction of a compound, Ti2InB2, a stable boron-based ternary phase in the Ti-In-B system, using a computational structure search strategy. This predicted Ti2InB2 compound is successfully synthesized using a solid-state reaction route and its space group is confirmed as P\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 6$$\end{document}6¯m2 (No. 187), which is in fact a hexagonal subgroup of P63/mmc (No. 194), the symmetry group of conventional Mn+1AXn phases. Moreover, a strategy for the synthesis of MXenes from Mn+1AXn phases is applied, and a layered boride, TiB, is obtained by the removal of the indium layer through dealloying of the parent Ti2InB2 at high temperature under a high vacuum. We theoretically demonstrate that the TiB single layer exhibits superior potential as an anode material for Li/Na ion batteries than conventional carbide MXenes such as Ti3C2.
... Experiments [68,123,143,144]. Differences in elastic properties between Si-FG-1 and Al-FG-1 are reported to result from the difference in strength between Ti-Si and Ti-Al bonds [8,9,145]. ...
Thesis
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Les phases MAX sont des carbures et / ou des nitrures ternaires avec un fort potentiel dans des applications diverses. Cette étude a porté sur deux phases MAX, Ti3SiC2 et Ti2AlC qui sont les plus connues dans ce groupe de matériaux. La première partie de ces travaux était dédiée à l’élaboration des poudres et des matériaux frittés. L’objectif était d’obtenir une variété de matériaux présentant différentes caractéristiques microstructurales, en termes de composition chimique et de taille de grains. Ainsi, des poudres commerciales et synthétisées par SHS ont été densifiées à l'aide de deux techniques de frittage sous charge, i.e. SPS et HP. La deuxième partie du travail a été consacrée à une meilleure compréhension de l’influence de la composition chimique et de la taille des grains sur le comportement thermomécanique des phases MAX. Des informations supplémentaires ont été fournies en couplant deux techniques expérimentales, la flexion quatre points et l’émission acoustique, et en les associant à des observations SEM post-mortems. L’approche expérimentale développée, basée sur la comparaison des réponses mécaniques des matériaux Ti3SiC2 (contenant la phase MAX et des phases secondaires) et de Ti2AlC (phases MAX uniquement), a permis d’approfondir la compréhension des mécanismes de déformation et d’endommagement induits. Il était également montré que les phases sécondaires et la taille de grains influence la manière dont les différents endommagements sont accumulées dans le matériaux. Les résultats d’EA sont fourni les informations supplémentaires sur les type d’endommagements rencontrées et leur chronologie qui résultent avec le comportement nonlinéaire de phases MAX. La dérnière partie de cette thèse a montré que le température de transition fragile-plastique est autour de 1200˚C et que la taille de grains l’abaisse.
... (40 μ cm). 11,35 The values obtained on the bulk sample are larger. This implies that defects limit the mean free path at low temperature. ...
Article
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The anisotropy of Ti2AlC transport properties is investigated focusing on the Hall effect and resistivity vs temperature measurements performed on a highly (000l)-oriented thin film and a bulk polycrystalline sample. Experimental data are interpreted on the basis of density functional theory calculations including transport coefficients obtained with the Boltzmann semiclassical transport equation in the isotropic relaxation time approximation. It is shown that the Hall constant is independent of the temperature and that the charge-carrier sign depends on the investigated crystallographic orientation. Charge carriers exhibit a holelike character along the basal plane of the Ti2AlC, whereas the bulk sample Hall constant is negative. The resistivity anisotropy is also evidenced: using an effective medium approach, the room temperature basal plane resistivity is shown to be more than one order of magnitude lower than that along the c axis. This very important anisotropy is shown to result from the anisotropy of the Fermi surface increased by electron-phonon interactions. These interactions are much more important along the c axis than within the basal plane, a situation opposite to that observed in literature for Ti2GeC where resistivity was reported to be isotropic.
... [5][6][7]14 Despite being an important parameter, there are only a few theoretical/experimental studies reported on this topic, 13,15−21 and a fundamental understanding of the interaction between the M n+1 X n -layers and the A-layers is not yet established. Calculations of partial density of states indicate bonding between Ti 3d and Al 3p in Ti 3 AlC 2 , see, e.g., Zhou et al. 13 The strength and nature of this bonding have generally been suggested to be weak and even of weak covalent character; 19,20 however, conclusive evidence thereof remains to be presented. ...
Article
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The inherently nanolaminated Ti3AlC2 is one of the most studied MAX-phase materials. MAX-phases consists of two-dimensional Mn+1Xn-layers (e.g., T3C2-layers) with strong internal covalent bonds separated by weakly interacting A-layers (e.g., Al-layers), where the repetitive stacking of the Mn+1Xn-layers and the A-layers suggests being the foundation for the unusual but attractive material properties of the MAX-phases. Although being an important parameter, the nature of the bonding between the Mn+1Xn-layers and the A-layers has not yet been established in detail. The X-ray photoelectron spectroscopy data presented in this paper suggest that the weak interaction between the Ti3C2-layers and the Al-layers in Ti3AlC2 is through electrostatic attraction facilitated by a charge redistribution of the delocalized electrons from the Ti3C2-layers to the Al-layers. This charge redistribution is of the same size and direction as between Ti atoms and Al atoms in TiAl alloy. This finding opens up a pathway to predict and improve MAX-phase materials properties through A-layer alloying, as well as to predict new and practically feasible MXene compounds.
... However, the Al-Ti bond has a larger bond length (2.89517 Å) as well as the charge between them has no directionality. It can be then concluded that the Al-Ti bond does not belong to a pure covalent bond, and a weak bond such as Van der Waals force occurs between them [35]. After the substitution of Ag for Al, the Ti-Ag bond will be produced. ...
Article
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The present work introduced first-principles calculation to explore the substitution behavior of Ag atoms for Al or Ti atoms in the Ti2AlC MAX phase ceramic. The effect of Ag substitution on supercell parameter, bonding characteristic, and stability of the Ti2AlC was investigated. The results show that for the substitution of Ag for Al, the Al-Ti bond was replaced by a weaker Ti-Ag bond, decreasing the stability of the Ti2AlC. However, the electrical conductivity of the Ti2AlC was enhanced after the substitution because of the contribution of Ag 4d orbital electrons toward the density of states (DOS) at the Fermi level coupled with the filling of Ti d orbital electrons. For the substitution of Ag for Ti, new bonds, such as Ag-Al bond, Ag-C bond, Al-Al bond, Ti-Ti anti-bond, and C-C anti-bond were generated in the Ti2AlC. The Ti-Ti anti-bond was strengthened as well as the number of C-C anti-bond was increased with increasing the substitution ratio of Ag for Ti. Similar to the substitution of Ag for Al, the stability of the Ti2AlC also decreased because the original Al-Ti bond became weaker as well as the Ti-Ti and C-C anti-bonds were generated during the substitution of Ag for Ti. Comparing with the loss of Ti d orbital electrons, Ag 4d orbits contributed more electrons to the DOS at the Fermi level, improving the electrical conductivity of the Ti2AlC after substitution. Based on the calculation, the substitution limit of Ag for Al or Ti was determined. At last, the substitution behavior of Ag for Al or Ti was compared to discriminate that Ag atoms would tend to preferentially substitute for Ti atoms in Ti2AlC. The current work provides a new perspective to understand intrinsic structural characteristic and lattice stability of the Ti2AlC MAX phase ceramic.
... The unit cell of the MAX phase is characterized by near close-packed M layers interleaved with the layers of a pure A-group elements and with the X atoms filling the octahedral sites. The Agroup elements are positioned at the center of a trigonal prism [12]. Currently, investigators are paying attention to theoretical modeling of the MAX phases. ...
Preprint
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From first principles electronic structure calculations, we unravel the evolution of structural, electronic, and magnetic properties of pristine, defected, and strained titanium nitride MXene with different functional groups (-F, -O, -H, and -OH). The formation and cohesive energies reveal their chemical stability. The MAX phase and defect free functionalized MXenes are metallic except for oxygen terminated (Ti 2 NO 2 ) one which is 100% spin polarized half-metallic ferromagnet. The spin-orbit coupling significantly influences the bare MXene (Ti 2 N) to exhibit Dirac topology and band inversion near the high symmetry directions and Fermi level. The strain effect sways the Fermi level thereby shifting it toward lower energy state under compression and toward higher energy state under tensile strain in Ti 2 NH 2 . The Ti 2 NO 2 exhibits exotic electronic structure and magnetic states not only in pristine but also in strained and defected structures. Its half-metallic nature changes to semi-metallic under 1% compression and it is completely destroyed under 2% compression. In single vacancy defect, its band structure remarkably transforms from half-metallic to semi-conducting with large band gap in 12.5% Ti, weakly semi-conducting in 5.5% Ti, and semi-metallic in 12.5% O. The 25% N defect changes it’s half-metallic characteristic to metallic. Further, the 12.5% Co substitution preserves it’s half-metallic character, whereas Mn substitution allows it to convert half-metallic characteristic into weak semi-metallic characteristic preserving ferromagnetism. However, Cr substitution converts half-metallic ferromagnetic state to half-metallic anti-ferromagnetic state. The understanding made here on collective structural stability, and electronic band structure, and magnetic phenomena in novel 2D Ti 2 N derived MXenes open up their possibility in designing them for synthesis and thereby taking to applications.
... % C. It is likely that the properties determined from the sputtering source will affect the possibilities to deposit epitaxial Ti 3 SiC 2 . 57 In addition, there is a peak at 2θ ≈ 30°, corresponding to the Nowotny phase Ti 5 Si 3 C x . Emmerlich et al. 18 reported growth of Ti 5 Si 3 C x at 700 °C by sputtering from elemental sources, and it has also been deposited by sequential growth by Vishnyakov et al. 29 At 850 °C, there are peaks at low angles [for clarity, see Fig. 5(a)] that originate from Ti 3 SiC 2 , Ti 4 SiC 3 , and the inter- grown Ti 7 Si 2 C 5 phase. ...
Article
We investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 oC as well as the influence of the sputtering power at 850 oC for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in angular distribution between C, Si and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at.% in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by X-ray photoelectron spectroscopy shows C-Ti and Si-C bonding in the Ti3SiC2 films and Si-Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 oC and with growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 oC. Four-point probe resistivity measurements give values in the range 120 to 450 mucro-Ohm-cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.
Article
The influence of Ti content on crystallinity and bonding of Cr–Ti–C thin films deposited by magnetron sputtering have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy. Our results show that binary Cr–C films without Ti exhibit an amorphous structure with two non-crystalline components; amorphous CrCx and amorphous C (a-C). The addition of 10–20 at.% Ti leads to the crystallization of the amorphous CrCx and the formation of a metastable cubic (Cr1−xTix)Cy phase. The observation was explained based on the tendency of the 3d transition metals to form crystalline carbide films. The mechanical properties of the films determined by nanoindentation and microindentation were found to be strongly dependent on the film composition in terms of hardness, elasticity modulus, hardness/elasticity ratio and crack development.
Article
We report first-principles spectroscopic calculation of core level excitations in five MAX-phase compounds. The spectra of Ti-K edges in Ti2AlC and Ti2AlN, C-K edge in Ti2AlC, N-K edge in Ti2AlN, and Nb-K edge in Nb2AlC are calculated and found to be in good agreement with reported experimental measurements. Based on this agreement, the Al-K and Al-L3edges in the same five phases plus the Cr-K and C-K edges in Cr2AlC and the C-K edge in Nb2AlC are calculated as theoretical predictions. We further analyze the anisotropy in the calculated spectra to gain additional insights on the structure-properties relationships in these MAX-phase compounds. These results are further discussed in the context of the local atomic environments of the M, A, and X elements in MAX-phase compounds and in relation to their fundamental electronic structures.
Article
The reaction process between Ti2AlC and Ag-Cu filler alloy was mentioned in our previous study. However, the reaction mechanism between Ti2AlC and filler alloy remained uncertain due to the existence of TiAl2, which was widely distributed in the dual-phase Ti2AlC substrate and exhibited intense reaction with Cu. In current research, pure-phase Ti2AlC was brazed to Cu using Ag-Cu filler alloy respectively at 850°C and 900°C for 10 min. First of all, to investigate the influence of TiAl2 on clarifying the reaction mechanism, Ti2AlC substrates with different component (single phase and dual phase) were joined to Cu at 850°C for comparison. However, in these joints, it was difficult to find any other reactant except for AlCu2Ti. Thus, the pure-phase Ti2AlC was brazed to Cu at 900°C, aiming to intensify the interaction between substrates and filler alloy. For characterizing the microstructure evolution in the joint, the typical region of the joint that contained all the reactants was selected and sliced by focused ion beam technology. Combining with transmission electron microscopy, all the decomposition products (e.g. Ti3AlC2 and TiC) in the joint were identified. Then the decomposition mechanism of Ti2AlC was clearly disclosed.
Article
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We report first-principles spectroscopic calculation of core level excitations in five MAX-phase compounds. The spectra of Ti-K edges in Ti2AlC and Ti2AlN, C-K edge in Ti2AlC, N-K edge in Ti2AlN, and Nb-K edge in Nb2AlC are calculated and found to be in good agreement with reported experimental measurements. Based on this agreement, the Al-K and Al-L3 edges in the same five phases plus the Cr-K and C-K edges in Cr2AlC and the C-K edge in Nb2AlC are calculated as theoretical predictions. We further analyze the anisotropy in the calculated spectra to gain additional insights on the structure-properties relationships in these MAX-phase compounds. These results are further discussed in the context of the local atomic environments of the M, A, and X elements in MAX-phase compounds and in relation to their fundamental electronic structures.
Article
The present study investigated the effect of brazing temperature and holding time on the microstructure and mechanical properties of Ti2AlC-copper joints brazed with Ag-Cu filler alloy at 800–900 °C for 10 min and held for different times at 850 °C. The representative structure of the joints was Ti2AlC/interaction area in the Ti2AlC substrate/brazing layer/diffusion area of the copper substrates/Cu. The brazing layer was composed of Cu-rich phases, AlCu2Ti intermetallic compounds and Ag[Cu] solid solutions. With increasing brazing temperature, more filler alloy was infiltrated into the ceramic substrate and reacted with Ti2AlC, accelerating the decomposition of Ti2AlC and the formation of Ti3AlC2, TiC and AlCu2Ti reaction phases. Moreover, more copper substrate was dissolved into the filler alloy; thus, more Cu-rich phase and AlCu2Ti were produced in the brazing layer. Moreover, the width of the brazing layer was reduced, while the widths of both the interaction area and diffusion area increased. As the brazing temperature was increased, the shear joint strength increased and then decreased. By prolonging the holding time, the widths of both the interaction area and the diffusion area were increased, while the width of the brazing layer was held constant at approximately 50 μm. Neither the structure nor the shear strength of the joints was influenced by the holding time. The optimum brazing parameters for joining Ti2AlC ceramic to copper with Ag-Cu filler alloy were 850 °C for 40 min. The joints obtained exhibit a maximum shear strength of 203.3 MPa.
Article
In this study, ab initio calculations based on Pseudo-Potential Density Functional Theory (PP-DFT) method are carried out in order to highlight the partial substitution effect of Rare Earth (RE) elements in the well-known 211-MAX phase of Ti2AlC. The considered elements are Y, Sc and RE = La, Ce, Pr, Nd, Sm, Eu, Gd leading to (Ti3/2RE1/2)AlC alloys. According to the obtained results, the (Ti3/2RE1/2)AlC alloys are significantly less compressible under uniaxial stress along x and z axes. They exhibit high resistance to shearing along <001> direction. In addition, the calculated heat capacity for (Ti3/2RE1/2)AlC alloys increases with respect to the temperature, a maximum is found in the temperature range 200–300 K. Localized states occur in (Ti3/2RE1/2)AlC alloys due to the f states filling of the rare earth elements. The magnetic moment of (Ti3/2RE1/2)AlC compounds increases according to 4fⁿ(n=2 for Ce to n=7 for Gd) filling. Our findings provide a theoretical database for new tunable properties of (Ti3/2RE1/2)AlC alloys.
Article
In the present study, Cu-Sn-Ti filler alloy with different content of Sn was used to join Ti2AlC ceramic and copper at 950 °C for 10 minutes. Effect of Sn content on the microstructure, mechanical property and electrical conductivity of the joints were investigated.The results indicate that the joint was comprised of five parts: copper substrate/ diffusion area in the copper substrate (Cu [Al, S solid solutions)/ brazing layer (Cu [Al, S+CuSn3Ti5)/ interaction area in the Ti2AlC substrate (Ti2AlC+ Cu [A+AlCu2Ti+TiC)/ Ti2AlC ceramic substrate.With the content of Sn element in the joint increasing, the filler alloy performed lower melting point and better fluidity during brazing. Thus partial filler alloy flowed out of the brazing seam, leading to the reduction of CuSn3Ti5 phases. Simultaneously, more Ti and Al diffused toward the Cu substrate, where a line of AlCu2Ti phases was formed. The maximum shear strength 158.5 MPa was obtained by using Cu80Sn10Ti10 (at.%) filler alloy, at which the joint strength was 71% of that of the Ti2AlC ceramic. The joint strength was deteriorated while the higher content of Sn was incorporated (>10 at.%), which was caused by the weak interfacial bonding between the substrates and the brazing layer. Besides, the electrical conductivity was decreased from 5.65×106 s/m to 4.99×106 s/m with increasing Sn content in the filler alloy.
Article
Generally, intercalation occurs when foreign atoms intercalate into multi-layer structures, while adsorption occurs when foreign atoms interact with monolayer structures or surfaces. We performed an investigation on the Mg intercalation into Ti2C building block (MXene) from first-principles simulation. We found that Mg can favorably intercalate into MXene, forming the stable compound Ti2MgC, which corresponds to the stage I in the Li intercalation into graphite. Based on the evaluation of the average cell potential and the energy barrier of Mg diffusion for the most energetically stable structure, our results suggest that Ti2MgC is a potential anode for Mg ion batteries.
Chapter
Introduction Response of Quasi-Single Crystals to Compressive Loads Response of Polycrystalline Samples to Compressive Stresses Response of Polycrystalline Samples to Shear Stresses Response of Polycrystalline Samples to Flexure Stresses Response of Polycrystalline Samples to Tensile Stresses Hardness Fracture Toughness and R-Curve Behavior Fatigue Resistance Damage Tolerance Micromechanisms Responsible for High K1c, R-Curve Behavior, and Fatigue Response Thermal Sock Resistance Strain Rate Effects Solid Solution Hardening and Softening Machinability Summary and Conclusions References
Article
Development of fine grain 316L with small amount of TiC for high radiation-tolerant performance was tried considering the fabrication process of thermo-mechanical treatments. The materials obtained are UFG316L+2.0 mass% TiC with the grain size of 90–270 nm, depending on the final annealing temperature from 700 ℃ to 900 ℃. The materials were examined by transmission electron microscopy observation, X-ray absorption near-edge structure spectroscopy, high voltage electron microscope electron irradiation and stress corrosion cracking by crevice bent beam method in high-temperature water with 8 ppm dissolved oxygen. The test results showed that the material is generally of excellent quality. Especially void swelling induced by the electron irradiation at 400 ℃ is less than 1/10 compared to the commercial SUS316L.
Article
We report on the elaboration and transport properties of a sandwich like 2-dimensional Ti3C2Tx MXene/ Graphene composite through alternating electrospray of MXene and graphene materials. The structural and electrical properties were systematically investigated with respect to the graphene content. The surface roughness of the samples has found to decrease considerably after the graphene integration. Electrical measurements show a clear trend to increase in both electrical conductance and Hall carrier mobility with respect to the graphene concentrations, and even reach the values of 9.5 104 S/cm and 54.58 cm2 /V s, respectively, for only 2.5 wt. % of graphene, rendering this MXene based composite one of the most electrically conductive to date.
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Density functional theory is used to provide theoretical insights into the ternary nanolaminated and layered transition metal boride (MAB phase) of MoAlB, with calculations of crystal structure, electronic structure, lattice dynamics and elastic properties, including a corresponding hypothetical MAX phase compound Mo2AlC for comparison. The calculated atomic configuration matches well with experiment. The metal-like electronic structure contributes to the physical origin of the high electrical conductivity of MoAlB. Strong covalent bonding is present between the B atoms, as well as between the Mo and B atoms, and significantly the much weaker Al-Al bonds are consistent with the high fracture toughness and damage tolerance seen in MoAlB. With increasing pressure, the shrinkage is highest along the b axis, and lowest along the c axis. From the calculated second-order elastic constants, the bulk moduli B, shear moduli G, Young’s moduli E and Poisson ratio μ are 207 GPa, 137 GPa, 336 GPa and 0.23, respectively. The G/B ratio of 0.66—similar in magnitude to values in MAX phases –demonstrate similarities in properties between MAB and MAX phases. Lattice dynamics are examined in detail, with 9 Raman-active modes and 6 infrared-active modes identified and analyzed in terms of their atomic motion and wavenumbers.
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The Mn+1AXn phases (MAX phases for short, M: transition metal, A: A group elements, X: C or N, and n = 1-3) have attracted much attention due to the unique combination of the ceramic- and metal-like properties. The density functional theory (DFT) has emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of them with the interesting properties. After the beginning with a brief introduction of the MAX phase and DFT, we review the DFT study on this class of materials, including crystal structure, electronic structure, point defects, lattice dynamics and related properties, phase stability, compressibility, and elastic properties. Comparison between the theoretical values and available experimental ones shows that they are in decent agreement for most part, especially in the lattice constants, elastic properties, and compressibility. At the end of this paper, this review is concluded with an outlook of future research on DFT study of MAX phases, major challenges to be met and possible solutions in some cases.
Article
This paper presents a comparative study on the Ti2AlC coatings produced by different thermal spray methods, as Ti2AlC is one of the most studied materials from the MAX-phase family. Microstructural analysis of coatings produced by High Velocity Air Fuel (HVAF), Cold Spray and High Velocity Oxygen Fuel (HVOF) has been carried out by means of the scanning electron microscopy equipped with an energy dispersive spectrometer (EDS). The volume fraction of porosity was determined using the ASTM standard E562. The phase characterization of the as-received powder and as-sprayed coatings was conducted using the X-ray diffraction with CrKα radiation. Impact of the spray parameters on the porosity and the mechanical properties of the coatings are discussed. The results show that the spraying temperature and velocity play a crucial role in coatings characteristics.
Article
MAX phases are technologically important materials exhibiting both metallic and ceramic properties. In the present study we propose the use of the special quasirandom structure (SQS) approach as a computationally tractable method to predict the phase stability of disordered MAX phase solid solutions. We have generated 128-atom SQS structures to mimic the 211 MAX phase solid solutions with random distribution of different elements within either the M or the A sublattice. Using DFT-calculated mixing energy and instability energy as predictors, we show that (Zr1-xMx)2AlC (for M = Nb and Ta) and Zr2(Al1-xAx)C (for A = Bi, Pb and Sn) MAX phase solid solutions may be experimentally synthesized. Our predicted results are in agreement with the limited available experimental data and chemical bonding analysis using the crystal-orbital Hamilton population (COHP) technique. The SQS cells reported are transferable and can be employed to model numerous MAX phase solid solutions.
Article
Dynamical properties of the two-dimensional Ti2C and Ti2N MXenes were investigated using density functional theory and discussed in connection with their structures and electronic properties. To elucidate the influence of magnetic interactions on the fundamental properties of these systems, the nonmagnetic, ferromagnetic and three distinct antiferromagnetic spin arrangements on titanium sublattice were considered. Each magnetic configuration was also studied at two directions of the spin magnetic moment with respect to the MXene layer. The zero-point energy motion, following from the phonon calculations, was taken into account while analyzing the energetic stability of the magnetic phases against the nonmagnetic solution. This contribution was found not to change a sequence of the energetic stability of the considered magnetic structures of Ti2X (X = C, N) MXenes. Both Ti2X (X = C, N) systems are shown to prefer antiferromagnetic arrangement of spins between Ti layers and the ferromagnetic order within each layer. This energetically privileged phase is semiconducting for Ti2C and metallic for Ti2N. The type of magnetic order as well as the in-plane or out-of-plane spin polarizations have a relatively small impact on the structural parameters, Ti-X bonding length, force constants and phonon spectra of both Ti2X systems, leading to observable differences only between the nonmagnetic and any other magnetic configurations. Nonetheless, a noticeable effect of the spin orientation on degeneracy of the Ti-3d orbitals is encountered. The magnetic interactions affect to a great extent the positions and intensities of the Raman-active modes, and hence one could exploit this effect for experimental verification of the theoretically predicted magnetic state of Ti2X monolayers. Theoretical phonon spectra of Ti2X (X = C, N) MXenes exhibit a linear dependence on energy in the long-wavelength limit, which is typical for a 2D system.
Article
The development of renewable energy conversion and storage devices, aiming at high efficiency, stable operation, environmental friendliness, and low-cost goals, provides a promising approach to resolve the global energy crisis. Recently, two-dimensional (2D) layered materials have drawn enormous attention due to their unique layered structure and intriguing electrical characteristics, which brings the unprecedented board applications in the fields ranging from electronic, optical, optoelectronic, thermal, magnetic, quantum devices to energy storage and catalysis. Graphene-based 2D layered materials show promising applications in energy storage and conversion owing to their high specific surface area, which have been used for supercapacitor electrode materials based on the electrical double-layer capacitance model. However, graphene has a limited value of theoretical electrical double-layer capacitance when the whole surface area is fully utilized. Among several classes of 2D layered materials beyond graphene, transition metal dichalcogenides, transition metal carbides, and nitrides may exhibit excellent electrochemical properties due to the distinctive features of these 2D materials, such as large specific surface area, good hydrophilic nature, highly exposed active edge sites, and ease of intercalation and modification. Therefore, careful design and construction of these 2D compounds make them become potential candidates used for electrochemical supercapacitors and electrocatalytic hydrogen evolution. This review emphasizes the recent important advances of the 2D layered materials composed of transition metal dichalcogenides, transition metal carbides, and nitrides for supercapacitors and electrocatalysts. Furthermore, we discuss the challenges and perspectives in this energy field in terms of the classes of two-dimensional layered materials.
Article
The crystal structure, stability, elastic constants and electronic structures of a class of ternary transition-metal borides with the general formula of h-M2AB2 (where M is a 3d, 4d and 5d transition metal; A is Al, Ga, In; and h represents a hexagonal structure) were investigated using the first-principles calculations within the framework of density functional theory (DFT). The stability of the h-M2AB2 was assessed with the cohesive energy, formation energy and phonon dispersion curves. In general, h-M2AB2 crystallizes in a hexagonal structure, and consists of six-member boron rings (B6) and interleaved M and A layers. The h-M2AB2 containing an IVB-M and VB-M is more energetically favorable with respect to the orthorhombic MAB phase. The lattice constants and bond lengths of the ternary borides are highly dependent on their chemical composition. All the stable borides exhibit metallic transport behaviors. The solid B-B bonding is responsible to the good stability and high bulk modulus of h-M2AB2. The high ductility of late-transition-metal borides is attributed to the delocalized M d and A p electrons. The correlations between the elastic property and the electronic structure of h-M2AB2 were also discussed.
Article
Change in the mechanical properties of (Ti,Zr)(C,N) coatings by laser carburization was investigated in terms of bonding state and valence electron concentration (VEC). The doped carbon was analyzed for the formation of amorphous carbon phase with ID/IG of 0.85 and sp²/sp³ ratio of 2.13. The formation of carbide phase (TiC and ZrC) was substituted for nitrogen. The ratios of TiC/TiN and ZrC/ZrN were verified to be 0.10 and 0.26 by the analysis of the bonding environment using photoelectron spectroscopy. VEC was calculated to be 8.85 due to the transformation from (Ti,Zr)N (VEC = 9) to (Ti,Zr)(C,N). The decreased VEC was correlated with the decrease of the metallic state and the increase of p-d hybridized bond in valence band. As the measured elastic modulus and hardness increased from 420.3 GPa and 32.4 GPa for VEC = 9–442.1 GPa and 37.6 GPa for VEC = 8.85 before and after carbon doping, the change in resistive properties against the elastic strain was identified due to the increase in hybridized bonds.
Article
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The phase stability of Ti2AlC upon oxygen incorporation has been studied by means of first-principles calculations. Recent experimental observations of this so-called MAX phase ( M=early transition metal, A=A -group element, and X=C or N) show that the characteristic nanolaminated structure is retained upon oxygen incorporation, with strong indications of O substituting for C. Therefore, a solid solution of C and O on the carbon sublattice has been simulated by the so-called special quasirandom structure method. Through a developed systematic approach, the enthalpy of formation of Ti2Al(C1-x,Ox) has been compared to all experimentally known competing phases, and has been found favorable for all C to O ratios at the composition of the MAX phase. A negative isostructural formation enthalpy has also been predicted for Ti2Al(C1-x,Ox) . Altogether, the results indicate that a large amount of oxygen, at least up to x=0.75 , might be present in the Ti2AlC MAX-phase structure without decomposition of the material into its competing phases. Furthermore, an effect of an increased oxygen content is a corresponding increase in the bulk modulus and a change in electronic properties. These results are of importance for further understanding and identification of possible composition range of the MAX-phase oxycarbide, and hence for the prospect of tuning the material properties by a varying incorporation of oxygen.
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The electronic structure and chemical bonding of the layered ternary compounds Ti2AlC and Ti2AlN have been calculated by the ab initio pseudopotential total-energy method. The results show that Ti2AlC and Ti2AlN exhibit metallic electrical conductivity with an anisotropic character and Ti2AlC should be more conductive than Ti2AlN. The chemical bonding in Ti2AlC and Ti2AlN is also anisotropic and is metallic-covalent-ionic in nature. On the basis of total-energy estimation we conclude that the replacement of C by N will result in a stabilization of the hexagonal structure and decrease in metallic properties.
Article
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The electronic structure of the silicocarbide Ti3SiC2 has been determined by the full-potential linear-muffin-tin-orbital (FLMTO) method. The spectra of the core-electron levels and valence bands of Ti3SiC2 have been obtained by x-ray photoelectron spectroscopy (XPS) and compared with the results of FLMTO calculations and x-ray-emission spectroscopy (XES) data. Using XPS data of the inner electron levels (Ti 2p3/2, Si 2p, and O 1s) and the results of band calculations, the nature of chemical bonding in the silicocarbide was analyzed. The high plasticity of Ti3SiC2 is explained by a weak interaction between the layers comprising Ti6C octahedra and plane nets composed of silicon atoms. The electronic and cohesive energy properties of the nonstoichiometric Ti3SiC and hypothetical Ti3SiC2-based solid solutions (SS's), namely Ti3SiCN and Ti3SiCO, were simulated by the FLMTO method. An analysis of the cohesive properties shows probable destabilization of the hexagonal structure of Ti3SiC2 in the presence of C vacancies and oxygen impurities. By contrast, the partial substitution of N for C (Ti3SiCxN1-x SS's) should lead to an increase in the cohesive properties of the crystal.
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We have grown single-crystal thin films of Ti2GeC and Ti3GeC2 and a new phase Ti4GeC3, as well as two new intergrown MAX-structures, Ti5Ge2C3 and Ti7Ge2C5. Epitaxial films were grown on Al2O3(0001) substrates at 1000 °C using direct current magnetron sputtering. X-ray diffraction shows that Ti–Ge–C MAX-phases require higher deposition temperatures in a narrower window than their Ti–Si–C correspondences do, while there are similarities in phase distribution. Nanoindentation reveals a Young’s modulus of 300 GPa, lower than that of Ti3SiC2. Four-point probe measurements yield resistivity values of 50–200 μΩcm. The lowest value is obtained for phase-pure Ti3GeC2(0001) films.
Article
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Thin films of Mn+1AXn layered compounds in the Ti-Si-C system were deposited on MgO(111) and Al2O3(0001) substrates held at 900°C using dc magnetron sputtering from elemental targets of Ti, Si, and C. We report on single-crystal and epitaxial deposition of Ti3SiC2 (the previously reported MAX phase in the Ti-Si-C system), a previously unknown MAX phase Ti4SiC3 and another type of structure having the stoichiometry of Ti5Si2C3 and Ti7Si2C5. The latter two structures can be viewed as an intergrowth of 2 and 3 or 3 and 4 M layers between each A layer. In addition, epitaxial films of Ti5Si3Cx were deposited and Ti5Si4 is also observed. First-principles calculations, based on density functional theory (DFT) of Tin+1SiCn for n=1,2,3,4 and the observed intergrown Ti5Si2C3 and Ti7Si2C5 structures show that the calculated difference in cohesive energy between the MAX phases reported here and competing phases (TiC, Ti3SiC2, TiSi2, and Ti5Si3) are very small. This suggests that the observed Ti5Si2C3 and Ti7Si2C5 structures at least should be considered as metastable phases. The calculations show that the energy required for insertion of a Si layer in the TiC matrix is independent of how close the Si layers are stacked. Hardness and electrical properties can be related to the number of Si layers per Ti layer. This opens up for designed thin film structures the possibility to tune properties.
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We report on the synthesis and characterization of epitaxial single-crystalline Ti3SiC2 films (Mn+1AXn-phase). Two original deposition techniques are described, (i) magnetron sputtering from Ti3SiC2 compound target and (ii) sputtering from individual titanium and silicon targets with co-evaporated C60 as carbon source. Epitaxial Ti3SiC2 films of single-crystal quality were grown at 900 °C with both techniques. Epitaxial TiC(111) deposited in situ on MgO(111) by Ti sputtering using C60 as carbon source was used to nucleate the Ti3SiC2 films. The epitaxial relationship was found to be Ti3SiC2(0001)//TiC(111)//MgO(111) with the in-plane orientation Ti3SiC2[100]//TiC[101]//MgO[101]. © 2002 American Institute of Physics.
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The electronic structures of epitaxially grown films of Ti(3)AlC(2), Ti(3)SiC(2), and Ti(3)GeC(2) have been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured high-resolution Ti L, C K, Al L, Si L, and Ge M emission spectra are compared with ab initio density-functional theory including core-to-valence dipole matrix elements. A qualitative agreement between experiment and theory is obtained. A weak covalent Ti-Al bond is manifested by a pronounced shoulder in the Ti L emission of Ti(3)AlC(2). As Al is replaced with Si or Ge, the shoulder disappears. For the buried Al and Si layers, strongly hybridized spectral shapes are detected in Ti(3)AlC(2) and Ti(3)SiC(2), respectively. As a result of relaxation of the crystal structure and the increased charge-transfer from Ti to C, the Ti-C bonding is strengthened. The differences between the electronic structures are discussed in relation to the bonding in the nanolaminates and the corresponding change of materials properties.
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Epitaxial Ti3SiC2(0001) thin films have been deposited by dc magnetron sputtering from three elemental targets of Ti, C, and Si onto MgO(111) and Al2O3(0001) substrates at temperatures of 800-900 degreesC. This process allows composition control to synthesize M(n+1)AX(n) (MAX) phases (M: early transition metal; A: A-group element; X: C and/or N; n=1-3) including Ti4SiC3. Depositions on MgO(100) substrates yielding the Ti-Si-C MAX phases with (10 (1) over bar5), as the preferred orientation. Samples grown at different substrate temperatures, studied by means of transmission electron microscopy and x-ray diffraction investigations, revealed the constraints of Ti3SiC2 nucleation due to kinetic limitations at substrate temperatures below 700 degreesC. Instead, there is a competitive TiCx growth with Si segregation to form twin boundaries or Si substitutional incorporation in TiCx. Physical properties of the as-deposited single-crystal Ti3SiC2 films were determined. A low resistivity of 25 muOmega cm was measured. The Young's modulus, ascertained by nanoindentation, yielded a value of 343-370 GPa. For the mechanical deformation response of the material, probing with cube corner and Berkovich indenters showed an initial high hardness of almost 30 GPa. With increased maximum indentation loads, the hardness was observed to decrease toward bulk values as the characteristic kink formation sets in with dislocation ordering and delamination at basal planes.
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We have investigated the elastic properties of nanolayered M 2 AC, with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and A = Al, Ga, Ge, Sn, by ab initio calculations. We suggest that M 2 AC can be classified into two groups: One where the bulk modulus of the binary MC is conserved and another group where the bulk modulus is de-creased. This classification can be understood in terms of coupling between MC and A layers, which is defined by the valence electron population. These results may have implications for the understanding of properties and the performance of this class of solids.
Article
Local density functional calculations are used to address the electronic structures and the properties of chemical bonding of two definite phases formed within the ternary system Ti, Al and C: Ti2AlC and Ti3AlC. From the analyses of the density of states and of the crystal orbital overlap populations of the respective phases within the ASW method the role of C is assessed. Moreover, the bonding within TiC is discussed concomitantly. These calculations are of interest in the composite field to understand the mechanisms of formation of new compounds at the matrix/reinforcement interface.
Article
In this paper we report on the fabrication and characterization of Nb2AlC (actual Nb:Al:C ratios: 52.5±0.5, 24.0±0.2 and 23.5±0.5 at.%, respectively) and (Ti,Nb)2AlC (actual Ti:Nb:Al:C atomic ratios: 24.4±0.5, 27.3±0.5, 24.0±0.3 and 24.4±0.5 at.%, respectively). Polycrystalline, fully dense, predominantly single-phase samples of Nb2AlC, (average grain size ≈14±2 μm) were fabricated by reactive hot isostatic pressing of Nb, graphite, and Al4C3 at 1600°C for 8 h and 100 MPa. The identical procedure resulted in predominantly single-phase samples - with an average grain size of 45 μm - of (Ti,Nb)2AlC. To obtain finer-grained (≈15±3 μm) samples of the solid solution the powder mixtures were hot pressed at 1450°C for 24 h. The a and c lattice parameters of Al-poor Nb2AlC samples are, respectively, 3.107±0.001 and 13.888±0.001 Å; the corresponding values for the solid solution are 3.077±0.001 and 13.790±0.001 Å. Since the hardness of the solid solution (5.8 GPa) is in between those of Nb2AlC (6.1 GPa) and Ti2AlC, and, at comparable grain sizes, Nb2AlC is stronger, we conclude that no solid solution strengthening occurs in this system. All samples explored in this work are quite damage tolerant and thermal shock resistant. A 300 N Vickers indentation in a 1.5 mm thick, four-point bend bar decreases the strengths by anywhere from 25 to 50% depending on grain size. Quenching in water from 1200°C reduces the four-point flexural strength by 40 to 70%; i.e., it is not catastrophic. Both compositions are strain rate sensitive. Like Ti3SiC2 and ice, and for the same reasons, the compressive stress of the Nb2AlC sample decrease with decreasing strain rate. In contradistinction, and for reasons that are not understood, the strengths of the (Ti,Nb)2AlC samples decrease with increasing strain rates. The grain growth kinetics of Nb2AlC are quite sluggish; no appreciable grain growth was observed even after annealing at 1600°C for 16 h.
Article
The Al L2,3 emission spectra of Al-Mn, Al-Co, Al-Ni and Al-Cu alloys with various composition of transition metals have been measured with a grazing incidence spectrometer. The results show that the Fermi edges of the Al L2,3 emission spectra of the alloys do not markedly shift with the change of phase, and that the energy levels arising from the 3d states of the transition metal and contributing to the formation of the conduction band of the alloy locate at a few electron volts below the Fermi edge in any phases of the aluminum-transition metal alloys.
Article
The Mn+1AXn layered carbide/nitride-derived phases, where M is an early transition metal, A is an A-group element and X is N or C, have an unusual combination of mechanical, electrical and thermal properties. The surface and crystal-chemistries of two members with n=2 and 3 have been investigated by X-ray photoelectron spectroscopy. The results show that the constituent species are characterized by low binding energies, sometimes exceptionally so. The energies are 281.0 281.5 and 396.9 eV, respectively, for C 1s and N 1s, both of which are at, or below, the lowest values measured for carbides and nitrides. Similarly the Ti 2p energies, in the range 454.0 454.7 eV, are comparable to that of Ti metal and TiC, while the energy of the Al 2p, 72.0 eV, is lower by ca. 0.8 eV than that for Al metal. The signature of the XTi6 octahedral units in the stacking sequence is reminiscent of the corresponding units in TiN, and it is found that a decrease in Ti 2p binding energy is correlated with decrease in average X-Ti bond length. The low binding energies of the A-type species, Al and Si, in planar coordination suggest that binding and thus screening may be derived from out-of-plane interactions. Results relevant to oxidation arising from exposure to air have been obtained. Yes Yes
Article
Local density functional calculations are used to address the electronic structures and the properties of chemical bonding of two definite phases formed within the ternary system Ti, Al and C:Ti2AlC and Ti3AlC. From the analyses of the density of states End of the crystal orbital overlap populations of the respective phases within the ASW method the role of C is assessed. Moreover,the bonding within TIC is discussed concomitantly. These calculations are of interest in the composite field to understand the mechanisms of formation of new compounds at the matrix/reinforcement interface.
Article
Transmission electron microscopy (TEM) of aligned, macrograined samples of Ti3SiC2, deformed at room temperature, shows that the deformed microstructure is characterized by a high density of perfect basal-plane dislocations with a Burgers vector of 1/3〈112 0〉. The dislocations are overwhelmingly arranged either in arrays, wherein the dislocations exist on identical slip planes, or in dislocations walls, wherein the same dislocations form a low-angle grain boundary normal to the basal planes. The arrays propagate across entire grains and are responsible for deformation by shear. The walls form as a result of the formation of kink bands. A dislocation-based model, that builds on earlier ideas proposed for kink-band formation in hexagonal metallic single crystals, is presented, which explains most of the microstructural features. The basic elements of the model are shear deformation by dislocation arrays, cavitation, creation of dislocation walls and kink boundaries, buckling, and delamination. The delaminations are not random, but successively bisect the delaminating sections. The delaminations and associated damage are contained by the kink boundaries. This containment of damage is believed to play a major role in endowing Ti3SiC2 and, by extension, related ternary carbides and nitrides with their damage-tolerant properties.
Article
A survey of some more recent results on the structural chemistry of compounds between transition elements and IVb group elements (carbon, silicon, germanium and tin) will be presented. There are essentially two large classes of compounds to be discussed, characterized by uniform structural principles, namely transition element carbides and related phases on the one hand and defect disilicide structure compounds and derivatives on the other.Starting with the problem of carbon ordering in transition element carbides and hydrogen containing carbides which reveal the significance of the octahedral [T6C]-group, numerous complex carbides of the general formula TxMyCz (T = transition element, M = another transition or B-group element) can be explained by means of a few common structural features. Perovskite carbides of formula T3MC, corresponding to the filled up Cu3Au-type or the filled up U3Si-type structures, β-Mn carbides of formula T3M2C, corresponding to the filled up β-Mn-type structure and K-carbides, related to the Mn3Al9Si-type structure are characterized by linking of the [T6C]-groups by corners. H-phase carbides of formula T2MC and carbides having Ti3SiC2-type structure exhibit linking of the [T6C]-groups by edges. A similar mode of linking also occurs for carbides with V3AsC-type or the filled Re3B-type structures, although in some cases such as VCr2C2 the trigonal prismatic [T6C]-group intervenes. Finally, the η-carbides having filled Ti2Ni-type and carbides of formula T5M3C with filled up Mn5Si3-type structure can be regarded as built up by linking of the octahedral [T6C]-groups by faces. The geometrical factor within the carbides is strongly supported by the short T-C-distances in the structural element [T6C], thus the formation and architecture of complex carbides may be understood from a topo-chemical point of view, for example, Ti2GeC (H-phase carbide) consists of the sum of TiC (octahedral group) and TiGe (trigonal prism).The second class of compounds, which are derived from the TiSi2-type structure, also belong to an uniform geometrical principle; however, some influence of the electronic concentration on the defect of the B-group element (Si,Ge) and the cell parameter will be recognized. The peculiar structural principle can be described by a partial lattice of the transition metal atom corresponding almost perfectly to that of the Ti-atoms of the TiSi2-type while the second partial lattice (Si,Ge or Sn) according to the defect of these atoms is expanding in one direction of the generating (110) plane. As a consequence of the mutual interference of T- and B-group element atoms a helicoidal structural element of the respective Si, Ge, etc., atoms results. Thus, the arrangement is characterized by a typical subcell and occasionally by extremely long c-axis. That also means, fairly complex compositions occur such as Mn11Si19, Mo13Ge23, V17Ge31 or Rh39 (Ga0.5Ge0.5)58. The problem of pseudo-homogenous domains of compounds arise in as far as within fairly small regions of composition a split according to different multiple of subcell will be observed, such as Mn11Si19(MnSi1.727); Mn26Si45(MnSi1.730); Mn15Si25(MnSi1.733) and Mn27Si47(MnSi1.741). A similar change in the multiple of subcells that means independent phases, takes place by substituting either the transition element by another or by substituting the B-group element by another B-group element such as Cr37Ge59As4 which corresponds to the Rh10Ga17-type while Cr11Ge19 is isotypic with Mn11Si19. In general, lowering of the overall electron concentration diminishes the defect of the B-group element in the compound.
Article
The ternary compounds Zr2TlC, Zr2PbC, Hf2TlC and Hf2PbC were prepared and examined. The crystal structure of these carbides has been found to be isotypic with Cr2AlC, H-phase.
Article
Electronic structures of the hexagonal Ti2AlC and Ti2AlN compounds with Cr2AlC-type structure calculated within the full-potential linearized augmented plane-waves formalism are presented. Geometrical optimization of the unit cell are in good agreement with experimental data. The analysis of the site and momentum projected densities of states shows that bonding is due to Ti d-C p (or Ti d-N p) and Ti d-Al p hybridizations. It is found that the intensity of the total density of state at Fermi level is higher for Ti2AlN that has also a higher electrical conductivity. Results are compared to a recent work by Zhou and Sun1 who assume a different crystal structure.
Article
We present sets of special points in the Brillouin zone from which the average over the Brillouin zone of a periodic function of wave vector (e.g., energy, charge density, dipole matrix elements, etc.) can be determined in a simple and accurate way once the values of the function at these points are specified. We discuss a method for generating the special-point sets and apply it to the case of crystals with cubic and hexagonal Bravais lattices.
Article
Thin films of the Mn+1AXn-phases Ti2AlC and Ti3AlC2 have been deposited by dc magnetron sputtering. In agreement with the Ti–Si–C system, the MAX-phase nucleation is strongly temperature dependent. At 900 °C epitaxial films of Ti2AlC and Ti3AlC2 were grown, but at 700 °C only a cubic (Ti,Al)C phase was formed. In addition, a perovskite carbide, Ti3AlC was grown at 800 °C. A bulk resistivity of 0.51 μΩ m, 0.44 μΩ m, and 1.4 μΩ m was measured for the Ti3AlC2, Ti2AlC, and Ti3AlC films deposited at 900 °C, respectively. By nanoindentation the hardness and Young’s module was determined for an epitaxial Ti3AlC2 film to 20 GPa and 260 GPa, respectively.
Article
A new tool for analysing theoretically the chemical bonding in solids is proposed. A balanced crystal orbital overlap population (BCOOP) is an energy resolved quantity which is positive for bonding states and negative for antibonding states, hence enabling a distinction between bonding and antibonding contributions to the chemical bond. Unlike the conventional crystal orbital overlap population (COOP), BCOOP handles correctly the situation of crystal orbitals being nearly linear dependent, which is often the case in the solid state. Also, BCOOP is much less basis set dependent than COOP. A BCOOP implementation within the full-potential linear muffin tin orbital method is presented and illustrated for Si, TiC and Ru. Thus, BCOOP is compared to the COOP and crystal orbital Hamilton population (COHP) for systems with chemical bonds ranging from metallic to covalent character.
Article
The electronic structure in the new transition-metal carbide Ti(4)SiC(3) has been investigated by bulk-sensitive soft x-ray emission spectroscopy and compared to the well-studied Ti(3)SiC(2) and TiC systems. The measured high-resolution Ti L, C K, and Si L x-ray emission spectra are discussed with ab initio calculations based on density-functional theory including core-to-valence dipole matrix elements. The detailed investigations of the Ti-C and Ti-Si chemical bonds provide increased understanding of the physical properties of these nanolaminates. A strongly modified spectral shape is detected for the intercalated Si monolayers due to Si 3p hybridization with the Ti 3d orbitals. As a result of relaxation of the crystal structure and the charge-transfer from Ti (and Si) to C, the strength of the Ti-C covalent bond is increased. The differences between the electronic and crystal structures of Ti(4)SiC(3) and Ti(3)SiC(2) are discussed in relation to the number of Si layers per Ti layer in the two systems and the corresponding change of materials properties.
Article
The deformation mechanisms in ductile Ti3SiC2(0001) single-crystal films have been analysed by nanoindentation and cross-sectional transmission electron microscopy. Permanent deformation includes formation of kink bands, as the nanolaminated material buckles out at the perimeter of the contact area, and delamination cracks. Evidence is presented for incipient kink-band formation.
Article
We report that magnetron sputtering can be applied to synthesize MAX-phase films of several systems including Ti–Si–C, Ti–Ge–C, Ti–Al–C, and Ti–Al–N. In particular, epitaxial films of the known phases Ti3SiC2, Ti3GeC2, Ti2GeC, Ti3AlC2, Ti2AlC, and Ti2AlN as well as the newly discovered thin film phases Ti4SiC3, Ti4GeC3 and intergrown structures can be deposited at 900–1000 °C on Al2O3(0001) and MgO(111) pre-seeded with TiC or Ti(Al)N. From XTEM and AFM we suggest a growth and nucleation model where MAX-phase nucleation is initiated at surface steps or facets on the seed layer and followed by lateral growth. Differences between the growth behavior of the systems with respect to phase distribution and phase stabilities are discussed. Characterization of mechanical properties for Tin+1Si–Cn films with nanoindentation show decreased hardness from about 25 to 15 GPa upon penetration of the basal planes with characteristic large plastic deformation with pile up dependent on the choice of MAX material. This is explained by cohesive delamination of the basal planes and kink band formation, in agreement with the observations made for bulk material. Measurements of the electrical resistivity for Ti–Si–C and Ti–Al–N films with four-point probe technique show values of 30 and 39 μΩ cm, respectively, comparable to bulk materials.
Article
The ternary compund Ti3SiC2 is a prominent representative of a new class of layered ceramics whose extraordinary physical properties has attracted much attention in recent years. Ti3SiC2 is electrically and thermally highly conductive, elastically rigid, lightweight, and maintains its strength to high temperatures. It is furthermore damage tolerant and oxidation resistant. We have studied fractured surfaces of coarse-grained Ti3SiC2 by means of photoelectron spectroscopy at the MAX-lab synchrotron radiation facility in Lund, Sweden. High-resolution C 1s, Si 2p, Ti 2p, Ti 3s and Ti 3p core-level spectra are reported and interpreted in terms of crystallographic and electronic structure. Valence band spectra confirm the validity of recent band calculations.
Article
Die Struktur von Ti3SiC2 wird aus Einkristallaufnahmen bestimmt. Die Gitterparameter der hexagonalen Zelle sind:a=3,068,c=17,669 undc/a=5,759. Die Titan-Atome besetzen die Punktlagen 2a) und 4f) (zTi=0,135), die Silicium-Atome die Punktlage 2b) und die Kohlenstoff-Atome die Punktlage 4f) (zC=0,5675) in der Raumgruppe D 6h 4 –P63/mmc. Die Struktur gehrt zu den Komplexcarbiden mit oktaedrischen Bauelementen [T 6C].The crystal structure of Ti3SiC2 has been determined by means of single crystal photographs; the lattice parameters of the hexagonal cell were found to be:a=3.068,c=17.669 andc/a=5.759. The titanium atoms occupy the positions 2a) and 4f) (zTi=0.135), the silicon atoms 2b) and the carbon atoms 4f) (zC=0.5675) of the space group D 6h 4 –P63/mmc. The crystal structure type belongs to the class of complex carbides having octahedral groups [T 6C].
Article
A formalism to compute x-ray spectra due to core excitations in metals by using single-particle band-structure techniques is presented and illustrated with a detailed calculation of the $K,L$, and $M$ emission and absorption spectra of palladium over 200 eV. Within the muffin-tin approximation for the potential, any spectrum can be factorized into atomiclike and solid-state contributions. The atomiclike factor is the dipole transition strength connecting a core state to a muffin-tin orbital in a free-electron metal. The solid-state factor is proportional to the density of band states with angular momentum determined by the orbital symmetry of the core state and the dipole selection rules. These projected densities of states have been calculated by using a linearized version of the augmented-plane-wave method specifically designed to cover large energy ranges. In particular, the method can describe simultaneously several principal quantum numbers of the eigenstates (e.g., $4d$ and $5d$ for palladium).
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
The new undulator beamline I511 at MAX-lab, now under commissioning, has been optimized for X-ray emission and photoelectron spectroscopies. Using an SX-700 high flux monochromator the accessible photon energy range is from 90 eV to about 1500 eV. The performance of the undulator agrees very well with the specifications, as shown by measurements using a photodiode. The energy resolution of the monochromator has been checked using absorption measurements in a gas cell. It was found to meet the expectations and exceeds a resolving power of 10 000 at 244 eV. The photon flux as a function of energy has been recorded as well and gives a maximum flux of 3×1013 photons/s/100 mA/0.1% BW. Beamlines I511 and I411 will be the first synchrotron beamlines making use of a so-called beam waist phenomenon, known from laser physics. We show results of ray-tracing calculations to determine the ultimate spot size on the sample location. The endstations to be used at this new beamline and their capabilities will be discussed as an example of the future use of this facility.
Article
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
The unusual magnetic behavior of the heavier Ce monopnictides may be understood on the basis of a model Hamiltonian for a system of moderately delocalized f states hybridizing with band states. The parameters entering the theory have previously been taken as phenomenological input. We present a first-principles calculation of the parameters in the model Hamiltonian based on self-consistent, warped–muffin-tin, linear muffin-tin-orbital (LMTO) band structures calculated for CeBi, CeSb, CeAs, and CeP. With the self-consistent potential, we calculate the bands and the band-f hybridization matrix element entering the Anderson lattice Hamiltonian. The band-f hybridization potential is derived from the 4f5/2 resonance in the potential surrounding a Ce site; the f-state energy with respect to the band Fermi energy and the f-f correlation energy U are estimated by averaging f-state eigenvalues of f0, f1, and f2 Ce configurations. The result is used to calculate the anomalous crystal-field splitting of the Ce 4f5/2 manifold predicted by the model Hamiltonian for the Ce monopnictides. Due to the structure of the cubic symmetry group, band-f hybridization has a greater effect on the Γ8 quartet than on the Γ7 doublet of the 4f5/2 manifold, and the reduction of the splitting of the crystal-field levels from that expected on extrapolation from the isostructural heavier rare-earth monopnictides may be understood quantitatively on this basis. Our quantitative results are in good agreement with experimental values. We also calculate the range functions describing the anisotropic magnetic behavior of CeBi and CeSb, in fair agreement with phenomenological parameters fitted to data on those materials.
Article
A full-potential linearized muffin-tin orbital calculation is presented of titanium-carbon systems in a variety of crystallographic forms. The calculated electronic structure, total energies, and equilibrium lattice constants are determined for the ground-state NaCl structure of TiC and for prototype superlattice structures, and these results are discussed in terms of the nature of bonding found in TiC. Similar calculations are also given for WC in two of these crystalline forms, and the differing ground-state structure and equilibrium lattice constants in these two carbide materials are related to the behavior of those metallic d states which are occupied in WC and unoccupied in TiC. The behavior of these one-electron states, which stabilize WC in a simple hexagonal form, is similar to the calculated behavior of associated states in the prototype superlattice Ti-C structures, and these states are found to play a similar role in determining the structural characteristics in these systems. Some of the properties and probable stability of the various crystalline forms are also discussed in terms of our results.
Article
Unoccupied energy bands of iron are mapped by inverse photoemission from Fe(100), Fe(110), and Fe(111). The ferromagnetic exchange splitting deltaEex of the uppermost d band is measured for the H'25 point, where the minority- and majority-spin subbands are both empty (deltaEex=1.8 eV with H'25↓ at 1.9 eV and H'25↑ at 0.12 eV above EF). Several other critical points are determined, such as the minority-spin Gamma12 and P3 points, the majority-spin N3 point, and the higher-lying H15,H1 points of s,p character. Critical points and exchange splitting are compared with first-principles, local-density calculations. The real part of the self-energy is obtained from this comparison, and the imaginary part by measuring the liftime broadening. In the d-band region, the self-energy causes a 10% compression of the bands and a linear broadening Gamma(E)~0.6\|E-EF\|.