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Anisotropy in the electronic structure of V2GeC investigated by soft x-ray emission spectroscopy and first-principles theory
Abstract
The anisotropy of the electronic structure of ternary nanolaminate V2GeC is investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured polarization-dependent emission spectra of V L2,3, C K, Ge M1, and Ge M2,3 in V2GeC are compared with those from monocarbide VC and pure Ge. The experimental emission spectra are interpreted with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Different types of covalent chemical bond regions are revealed: V 3d-C 2p bonding at −3.8 eV, Ge 4p-C 2p bonding at −6 eV, and Ge 4p-C 2s interaction mediated via the V 3d orbitals at −11 eV below the Fermi level. We find that the anisotropic effects are high for the 4p valence states and the shallow 3d core levels of Ge, while relatively small anisotropy is detected for the V 3d states. The macroscopic properties of the V2GeC nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work.
... 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]. ...
... The Ge M 2,3 peak splitting is 3.6 eV while the calculated ab initio spin-orbit splitting is 4.3 eV (Table 2). Moreover, the calculated shallow Ge 3d core levels are 3.9 eV closer to E F and 10 times more intense than in the experiment [46,23,26]. The difference can be attributed to screening and relaxation effects. ...
... A lower branching ratio is thus an indication of higher ionicity (resistivity) in the material. For the MAX phases, a higher branching ratio was observed by Magnuson et al. [25,16,26] in the basal planes indicating higher metallicity and conductivity than along the c-axis. ...
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 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]. ...
... The Ge M 2,3 peak splitting is 3.6 eV while the calculated ab initio spin-orbit splitting is 4.3 eV (Table 2). Moreover, the calculated shallow Ge 3d core levels are 3.9 eV closer to E F and 10 times more intense than in the experiment [46,23,26]. The difference can be attributed to screening and relaxation effects. ...
... A lower branching ratio is thus an indication of higher ionicity (resistivity) in the material. For the MAX phases, a higher branching ratio was observed by Magnuson et al. [25,16,26] in the basal planes indicating higher metallicity and conductivity than along the c-axis. ...
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 bottom panel in figure 4 shows experimental Ge M 2,3 XES spectra ( → d p 3 3 and → s p 4 3 transitions) of Cr 2 GeC aligned to the E F at the 3p 1/2 core level. Here, we find that the isotropic 4s-states of the Cr 2 GeC valence band have an exceptionally high intensity in a broad range from −20 eV up to the E F in comparison to other Ge-containing Ti 3 GeC 2 [37] and V 2 GeC [38] compounds. Contrary to the experimental observations on Ge in Cr 2 GeC, the calculated 4s valence band is more than 10 times weaker with most intensity in the range between −8 to −12 eV below E F . ...
... The shallow Ge 3d core levels at the bottom of the valence band between −30 and −35 eV also participate in the Cr-Ge bonding in Cr 2 GeC. The measured 3d 5/2, 3/2 spin-orbit splitting of ± 3.6 0.1 eV is consistent with the difference between the 3p 3/2, 1/2 and 3d 5/2, 3/2 spin-orbit splittings observed in XPS (4.1-0.5 eV) and in Ti 3 GeC 2 [37] and V 2 GeC [38]. Theoretically, the spin-orbit splitting is larger (4.4 eV) and the states are also about 4.5 eV closer to the E F than in the experiment. ...
... In the DFT calculations, the Ge 3d states are overestimated and not enough intensity is redistributed to the 4s valence band states. The same type of deficiency in DFT calculations is known also for Ge in Ti 3 GeC 2 [37] and V 2 GeC [38] as well as for Ga in GaN [39]. We clearly observe that the calculated 4s states at the bottom of figure 4 are too low in intensity, in particular at the E F . ...
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase CrGeC 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 CrGeC. The Cr , C K, and Ge , 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.
... XES + DFT) [e.g. 72,88,90,[97][98][99][100][101], is caused by a significant structural difference between the ab-axis and c-axis of the unit cell of MAX phases. This is expected to influence the physical, mechanical and technological properties as well, as further discussed in this Review. ...
... Consequently, theoretical modelling is an important tool for the exploration of the correlation of structure-mechanical properties in MAX phases. For example, the C 44 elastic modulus is an important indicator for damage tolerance of MAX phases because it reflects the resistance of the crystal to shear in the [010] or [100] plane along the (001) direction [72]. Sun et al. [73] have calculated the elastic behaviour of the c 44 co-efficient on the variation of M in M 2 AlC or M 2 GaC for the important M elements Ti, V and Cr. ...
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.
... XES + DFT) [e.g. 72,88,90,[97][98][99][100][101], is caused by a significant structural difference between the ab-axis and c-axis of the unit cell of MAX phases. This is expected to influence the physical, mechanical and technological properties as well, as further discussed in this Review. ...
... Consequently, theoretical modelling is an important tool for the exploration of the correlation of structure-mechanical properties in MAX phases. For example, the C 44 elastic modulus is an important indicator for damage tolerance of MAX phases because it reflects the resistance of the crystal to shear in the [010] or [100] plane along the (001) direction [72]. Sun et al. [73] have calculated the elastic behaviour of the c 44 co-efficient on the variation of M in M 2 AlC or M 2 GaC for the important M elements Ti, V and Cr. ...
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.
... As a consequence, even if DFT calculations predict that many MAX phases would show a quasi-2D electronic band structure (see, e.g., [3,10,17]), direct and reliable measurements of the band structure anisotropy are thus required in order to decorrelate effects due to the electronic structure from effects due to the relaxation time. Concerning electronic structure, anisotropy is most often determined by comparing spectra obtained by various techniques to those predicted by DFT calculations [18][19][20][21][22][23]. The anisotropy of polarization-dependent soft x-ray emission spectra was reported for thin layers of V 2 GeC [18], Ti 3 SiC 2 [19], and Cr 2 GeC [20]. ...
... Concerning electronic structure, anisotropy is most often determined by comparing spectra obtained by various techniques to those predicted by DFT calculations [18][19][20][21][22][23]. The anisotropy of polarization-dependent soft x-ray emission spectra was reported for thin layers of V 2 GeC [18], Ti 3 SiC 2 [19], and Cr 2 GeC [20]. It is directly due to the anisotropy of electronic structure, valence bonds, and hybridization. ...
We report an experimental and theoretical study of the electronic band structure and Fermi surfaces (FSs) of V2AlC single crystals using angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations. We provide evidence of the existence of equivalent, complex hole FSs with a nearly tubular form along the c axis, and of an intricate electron FS exhibiting tubes parallel to c connected to one another by smaller tubes with axes parallel to the ab plane. The electron FS thus exhibits a small delocalization along the c axis. The local orbital character of each observed band is experimentally and theoretically assessed. DFT calculations show an excellent agreement with experiment. We also report the observation of an unstable surface state.
... Soft X-ray absorption near-edge structure measurements were performed at the undulator beamline I511-3 at MAX II (MAX-lab Laboratory, Lund, Sweden), comprising a 49-pole undulator, and a modified SX-700 plane grating monochromator [34]. The measurements were performed with a base pressure lower than 5×10 −7 Pa. ...
The electronic structure of rhombohedral sp2 hybridized boron nitride (r-BN) is characterized by X-ray absorption near-edge structure spectroscopy. Measurements are performed at the boron and nitrogen K-edges (1s) and interpreted with first-principles density functional theory calculations, including final state effects by applying a core-hole. We show that it is possible to distinguish between different 2D planar polytypes such as rhombohedral, twinned rhombohedral, hexagonal and turbostratic BN by the difference in chemical shifts. In particular, the chemical shift at the B 1s-edge is shown to be significant for the turbostratic polytype. This implies that the band gap can be tuned by a superposition of different polytypes and stacking of lattice planes.
... Soft X-ray absorption near-edge structure measurements were performed at the undulator beamline I511-3 at MAX II (MAX-lab Laboratory, Lund, Sweden), comprising a 49-pole undulator, and a modified SX-700 plane grating monochromator. 34 The measurements were performed with a base pressure lower than 5× 10 −7 Pa. To enhance the π* contribution that is suppressed at normal incidence, 35 the XANES spectra were measured at 20°grazing incidence angle in total fluorescence yield (TFY) mode. ...
The electronic structure of rhombohedral sp2-hybridized
boron nitride (r-BN) is characterized by X-ray absorption near-edge structure spectroscopy. Measurements are performed at the boron and nitrogen K-edges (1s) and interpreted with f irst-principles density functional theory calculations, including final state effects by applying a core hole. We show that it is possible to distinguish between different 2D planar polytypes such as rhombohedral, twinned rhombohedral, hexagonal, and turbostratic BN by the difference in chemical shifts. In
particular, the chemical shift at the B 1s-edge is shown to be significant for the turbostratic polytype. This implies that the band gap can be tuned by a superposition of different polytypes and stacking of lattice planes.
... This scaling is necessary due to insufficient hybridization between the Zr 4s -N 2s orbitals at the bottom of the valence band that is inherent in the density functional theory (DFT). As similar bond situation with strong interaction at the bottom of the valence band has previously been observed in XES for shallow core levels of Ge [37] and Ga [15]. ...
The electronic structure, chemical bonding and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable Zr1-xAlxN (x=0.0, 0.12, 0.26, 0.40) is investigated by resonant inelastic X-ray scattering/X-ray emission and X-ray absorption spectroscopy and compared to first-principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first-principle all electron full-potential calculations where the core states are treated fully relativistic. As shown in this work, the bulk sensitivity and element selectivity of X-ray spectroscopy enables to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develop from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semi-coherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.
... This scaling is necessary due to insufficient hybridization between the Zr 4s -N 2s orbitals at the bottom of the valence band that is inherent in the density functional theory (DFT). As similar bond situation with strong interaction at the bottom of the valence band has previously been observed in XES for shallow core levels of Ge [37] and Ga [15]. ...
The electronic structure, chemical bonding, and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable Zr 1−x Al x N (x = 0.0, 0.12, 0.26, 0.40) are investigated by resonant inelastic x-ray scattering, x-ray emission, and x-ray absorption spectroscopy and compared to first principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first principles all-electron full-potential calculations where the core states are treated fully relativistically. As shown in this work, the bulk sensitivity and element selectivity of x-ray spectroscopy enables one to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develops from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semicoherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.
... The density of states (DOS) at E F is thus most often quite high [18]. Numerous previous works already describe features related to the electronic structure and some of its anisotropies, both on theoretical and experimental grounds (see, e.g., [28][29][30][31][32][33]). They are more exhaustively listed in a recent review article up to year 2017 [24]. ...
We report a study of the electronic structure of 211 MAX phases and how relatively simple rigid band models can equally describe the band structures (BSs) and Fermi surfaces (FSs) of those compounds. Rigid band structures were built up from Density Functional Theory (DFT) calculations, but their applicability and limits have been tested by comparing their outcomes with experimental results from Angle Resolved Photoemission Spectroscopy (ARPES) on Cr2AlC, V2AlC and Ti2SnC. The charge transfer between A, M, and X atoms was also assessed. A remarkable agreement is found between theory and experiment, therefore confirming the applicability of rigid band models to describe MAX phases electronic structure. From those results, we propose a new classification of the 211 MAX phases.
... The density of states (DOS) at E F is thus most often quite high [18]. Numerous previous works already describe features related to the electronic structure and some of its anisotropies, both on theoretical and experimental grounds (see, e.g., [28][29][30][31][32][33]). They are more exhaustively listed in a recent review article up to year 2017 [24]. ...
The nanolamellar M2AC or carbon-based “211 MAX” phases, where M is an early transition metal, A belongs to groups 13–15, and C is carbon, can be described by rigid band models. The same band model applies to all possible A elements belonging to a given group of the periodic table. Changing M for a given A is then equivalent to shifting the Fermi energy EF through a band structure common to all phases in the group. This is shown by comparing predictions of density functional theory (DFT) to angle-resolved photoemission spectroscopy (ARPES) measurements. In particular, the Fermi surface of a given Al-based MAX phase can be obtained with an acceptable degree of accuracy by simply selecting the appropriate ARPES isoenergy surface of another Al-based phase. In V2AlC, and in addition to conventional metal energy bands, both DFT and ARPES show the existence of a gapped nodal line located around 0.2 eV below EF or complex crossing points at EF with Dirac-like features in some directions. Application of the rigid band model suggests that these topological features as well as others, also predicted by DFT, can be positioned at or close to EF by an appropriate choice of M and A, or by using an appropriate combination of various M and A elements.
... 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]. The spectroscopic data (EELS, XES, or XAS) are generally compared to the local density of states (LDOS) obtained from band-structure calculations, which do not include the core-hole effect [36] inherently involved in such experiments. ...
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.
... 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. ...
... 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. ...
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.
... The top of the valence band and the bottom of the conduction band develop more C 2p and Ge 3p characters when including U and the Cr states hybridizes accordingly giving rise to smaller bandwidths. A similar behavior of Ge states was found on the V 2 GeC phase [35]. Since Cr 2 GeC is a metallic system, the DOS at the Fermi level is a key quantity for stability purposes. ...
... The anisotropy in the electronic structure and its effect on conduction are very difficult to probe experimentally given the fact that MAX-phase single crystals have only been synthesized as thin films [2] [7] [8] [9] [10] [11] or as small (< 0.2 mm) free-standing crystals [12], not in bulk. Therefore, only very few experimental studies on the anisotropy in electronic structure or electrical properties of MAX phases exist [2] [13] [14] [15] [16]. Nevertheless, the anisotropy is important. ...
We employ monochromatized electron energy loss spectroscopy to study Ti3SiC2 and Ti3AlC2. By probing individual grains aligned along different axes in bulk polycrystalline Ti3SiC2 and Ti3AlC2, this approach enables determination of the anisotropy of the dielectric functions and an estimate of the free-electron lifetime in different orientations. The dielectric functions are characterized by strong interband transitions in the low energy region. The energies plasmon resonance were determined to be and exhibit a strong orientation-dependence. Our measurements show that the free-electron lifetimes are also highly orientation-dependent. These results suggest that scattering of carriers in MAX phases is very sensitive to composition and orientation.
MXenes (transition metal carbides and nitrides/carbonitrides) are a novel type of two-dimensional (2D) materials that are derived from MAX-phase precursors. Their structural and compositional diversity, outstanding electrical conductivity, and tunable surface properties have made MXenes one of the best and most versatile families of 2D materials, with rapidly growing fields of application. Since their inception, MXenes have attracted enormous research interest in various research fields, including physics, chemistry, materials science, nanotechnology, nanomedicine, and environmental science. This chapter briefly introduces fundamental aspects of MXenes such as MAX-phase precursors, MXene derivatives, and methods of synthesis and processing. Various MXene properties such as electronic, optical, magnetic, and mechanical properties are then briefly discussed. The next section briefly describes the current advances of MXene-based composites in various application fields. Finally, the chapter highlights the current challenges and research prospects for the development of MXene-based advanced composite materials for specific applications.
This study demonstrates superior electrical and electroluminescence performance of inverted quantum-dot light-emitting diodes (QD-LEDs) with a V2O5/poly(N-vinylcarbazole) (PVK) hole conduction layer. Hole- and electron-only device measurements reveal a more balanced charge carrier injection as well as the higher hole conduction capability in the inverted QD-LED than the standard one. Smooth stepwise hole conduction energy levels with a remarkably reduced hole barrier height (Δh) from 1.74 to 0.89 eV at QD/PVK are found to be responsible for high hole conduction and high luminous efficiency in the inverted QD-LED, which is validated by ultraviolet photoelectron spectroscopy measurements. The down-shifted electronic energy levels of PVK for reducing the Δh are discussed from the point of view of molecular orientation of PVK governed by interfacial atomic interaction with underlayers of V2O5 and QD for standard and inverted device structures, respectively.
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.
This study investigates the effect of graphene oxide (GO) interlayer on electrical and electroluminescent (EL) performances of quantum-dot light emitting diodes (QD-LEDs) with poly(N-vinylcarbazole, PVK)/V2O5–x hole transport layers. The control QD-LEDs basically consist of multilayer heterojunctions of hole transport/injection PVK/V2O5–x bilayers, QD light emission layer, and electron transport ZnO nanoparticle layer, all of which are sequentially spin coated on indium-tin-oxide/glass substrates. The QD-LEDs with GO interlayer inserted between PVK and V2O5–x present superior electrical rectification and EL efficiency than those without GO interlayer. The hole-only devices with GO interlayer evidence higher hole conduction capability than those without GO by an order of magnitude. From ultraviolet photoelectron spectroscopy analysis, the hole transport enhancement in PVK/GO/V2O5–x heterojunctions is found to be responsible for reduced height of the highest hole barrier at QD/PVK interface from 1.74 to 0.75 eV by means of downshift of energy levels of PVK. Such energy level variation of PVK is discussed in terms of heterointerfacial orbital hybridization at PVK/V2O5–x, which are validated by diverse spectroscopies. The ability of GO interlayer to shift the PVK energy levels can be exploited for developing high-performance optoelectronics and electronics with hole transport organic/transition metal oxide heterojunctions.
The local structure and chemical bonding in two-phase amorphous Cr1−xCx nanocomposite thin films are investigated by Cr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies in comparison to theory. By utilizing the computationally efficient stochastic quenching (SQ) technique, we reveal the complexity of different Cr-sites in the transition metal carbides, highlighting the need for large scale averaging to obtain theoretical XANES and EXAFS spectra for comparison with measurements. As shown in this work, it is advantageous to use ab initio theory as an assessment to correctly model and fit experimental spectra and investigate the trends of bond lengths and coordination numbers in complex amorphous materials. With sufficient total carbon content (≥ 30 at%), we find that the short-range coordination in the amorphous carbide phase exhibit similarities to that of a Cr7C3±y structure, while excessive carbons assemble in the amorphous carbon phase.
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
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.
The electronic structure and chemical bonding of the recently discovered inverse perovskite Sc3AlN, in comparison to those of ScN and Sc metal, have been investigated by bulk-sensitive soft-x-ray emission spectroscopy. The measured Sc L, N K, Al L1, and Al L2,3 emission spectra are compared with calculated spectra using first-principles density-functional theory including dipole transition-matrix elements. The main Sc 3d–N 2p and Sc 3d–Al 3p chemical bond regions are identified at −4 and −1.4 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states in the Al L2,3 emission from Sc3AlN in comparison to that for pure Al metal is found, which reflects the Sc 3d–Al 3p hybridization observed in the Al L1 emission. The differences between the electronic structures of Sc3AlN, ScN, and Sc metal are discussed in relation to the change in the conductivity and elastic properties.
Phase-pure epitaxial thin films of (Ti,V)2GeC have been grown onto Al2O3(0001) substrates via magnetron sputtering. The c lattice parameter is determined to be 12.59 Å, corresponding to a 50/50 Ti/V solid solution according to Vegard’s law, and the overall (Ti,V):Ge:C composition is 2:1:1 as determined by elastic recoil detection analysis. The minimum temperature for the growth of (Ti,V)2GeC is 700 °C, which is the same as for Ti2GeC but higher than that required for V2GeC (450 °C). Reduced Ge content yields films containing (Ti,V)3GeC2 and (Ti,V)4GeC3. These results show that the previously unknown phases V3GeC2 and V4GeC3 can be stabilized through alloying with Ti. For films grown on 4H-SiC(0001), (Ti,V)3GeC2 was observed as the dominant phase, showing that the nucleation and growth of (Ti,V)n + 1GeCn is affected by the choice of substrate; the proposed underlying physical mechanism is that differences in the local substrate temperature enhance surface diffusion and facilitate the growth of the higher-order phase (Ti,V)3GeC2 compared to (Ti,V)2GeC.
The magnetic properties, electronic band structure and Fermi surfaces of the hexagonal Cr(2)GeC system have been studied by means of both generalized gradient approximation (GGA) and the +U corrected method (GGA + U). The effective U value has been computed within the augmented plane wave theoretical scheme by following the constrained density functional theory formalism of Anisimov and Gunnarsson (1991 Phys. Rev. B 45 7570-74). On the basis of our GGA + U calculations, a compensated antiferromagnetic spin ordering of Cr atoms has been found to be the ground-state solution for this material, where a Ge-mediated super-exchange coupling is responsible for an opposite spin distribution between the ABA stacked in-plane Cr-C networks. Structural properties have also been tested and found to be in good agreement with the available experimental data. Topological analysis of Fermi surfaces has been used to qualitatively address the electronic transport properties of Cr(2)GeC, and found an important asymmetrical carrier-type distribution within the hexagonal crystal lattice. We conclude that an appropriate description of the strongly correlated Cr d electrons is an essential issue for interpreting the material properties of this unusual Cr-based MAX phase.
Cr 2 GeC thin films were grown by magnetron sputtering from elemental targets. Phase-pure Cr 2 GeC was grown directly onto Al 2 O 3 (0001) at temperatures of 700–800 • C. These films have an epitaxial component with the well-known epitaxial relationship Cr 2 GeC(0001)//Al 2 O 3 (0001) and Cr 2 GeC(11 ¯ 20)//Al 2 O 3 (1 ¯ 100) or Cr 2 GeC(11 ¯ 20)//Al 2 O 3 (¯ 12 ¯ 10). There is also a large secondary grain population with (10 ¯ 13) orientation. Deposition onto Al 2 O 3 (0001) with a TiN(111) seed layer and onto MgO(111) yielded growth of globally epitaxial Cr 2 GeC(0001) with a virtually negligible (10 ¯ 13) contribution. In contrast to the films deposited at 700–800 • C, the ones grown at 500–600 • C are polycrystalline Cr 2 GeC with (10 ¯ 10)-dominated orientation; they also exhibit surface segregations of Ge as a consequence of fast Ge diffusion rates along the basal planes. The room-temperature resistivity of our samples is 53–66 μμcm. Temperature-dependent resistivity measurements from 15–295 K show that electron-phonon coupling is important and likely anisotropic, which emphasizes that the electrical transport properties cannot be understood in terms of ground state electronic structure calculations only.
The electronic structure and chemical bonding of wurtzite-GaN investigated by
N 1s soft x-ray absorption spectroscopy and N K, Ga M1, and Ga M2,3 emission
spectroscopy is compared to that of pure Ga. The measurements are interpreted
by calculated spectra using first-principles density-functional theory (DFT)
including dipole transition matrix elements and additional on-site Coulomb
interaction (WC-GGA+U). The Ga 4p - N 2p and Ga 4s - N 2p hybridization and
chemical bond regions are identified at the top of the valence band between
-1.0 and -2.0 and further down between -5.5 and -6.5 eV, respectively. In
addition, N 2s - N 2p - Ga 4s and N 2s - N 2p - Ga 3d hybridization regions
occur at the bottom of the valence band between -13 and -15 eV, and between
-17.0 and -18.0 eV, respectively. A band-like satellite feature is also found
around -10 eV in the Ga M1 and Ga M2,3 emission from GaN, but is absent in pure
Ga and the calculated ground state spectra. The difference between the
identified spectroscopic features of GaN and Ga are discussed in relation to
the various hybridization regions calculated within band-structure methods.
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.
We present results from first-principles calculations of ternary transition metal carbides in the M3SiC2 series (where M=early transition metal). We predict structural and mechanical properties of these new MN+1AXN phases. The bulk modulus of the ternary carbides, M3SiC2, are in the calculations found to be proportional to the bulk modulus of the corresponding binary carbides, MC. We have analyzed this behavior using a simple, nearest-neighbor bond model, as well as from first-principles total energy calculations and have found that it is caused by a considerably weaker M-Si bond compared to the M-C bond.
Ab initio calculations based on the density-functional pseudopotential approach have been used to study the electronic structure and chemical bonding in layered machinable Ti3SiC2 ceramic. The calculations reveal that all three types of bonding - metallic, covalent and ionic - contribute to the bonding in Ti3SiC2. The high electric conductivity is attributed to the metallic bonding parallel to the basal plane and the high modulus and high melting point are attributed to the strong Ti-C-Ti-C-Ti covalent bond chains in the structure.
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.
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.
V2GeC MAX-phase thin films were deposited by DC magnetron sputter epitaxy in the temperature range 450–850 °C. The MAX-phase nucleates directly on (0 0 0 l)-oriented sapphire-wafer substrates without the need for a seed layer. The films contain, however, a small fraction of binary vanadium carbide (VCx) inclusions. X-ray diffraction analysis furthermore shows that these inclusions partly consist of the ordered superstructure V8C7. The amount of Ge in the films decreases at higher temperatures, which can be attributed to Ge evaporation. At temperatures below 450 °C the films consist of polycrystalline Ge and an X-ray amorphous carbide phase attributed to VCx or V2C. No MAX-phase was observed in this temperature region. The electrical and mechanical properties of the films were characterized.
This paper deals with the bonding properties of 3d-, 4d-, and 5d-transition-metal carbides and nitrides. We consider NaCl-structure compounds, MC and MN, as well as carbides and nitrides with more complex crystal structures. The enthalpies of formation at zero temperature Delta0H(0) of the MC and MN compounds are taken from previous ab initio linear-muffin-tin-oribtals calculations. We describe the structure as formed by a metal fcc lattice in which nonmetal atoms have been inserted at interstitial positions. Delta0H(0) is divided into contributions from (i) formation of metallic fcc lattice, (ii) expansion of this lattice to the lattice spacing of the compound, and (iii) insertion of nonmetal atoms into the metal lattice. Delta0H(0), plotted versus the position of M in the d series, shows a pronounced maximum for the Ti, Zr, and Hf carbides. In agreement with other work, we interpret this as due to the filling of bonding p-d hybridized states. The maximum in Delta0H(0) is followed first by a decrease and then by an almost constant value. We interpret this as an effect of a gradual population of antibonding and nonbonding electronic states. Size effects, i.e., contribution (ii), are small for the 4d- and 5d-series compounds. Delta0H(0) of MN is similar to that of MC, but the largest values occur for M=Sc,Y,La since N has one more valence electron than C. Bonding in the complex carbides and nitrides is given an analogous interpretation.
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.
Superlattices of TiC/VC have been deposited on MgO(001) substrates by simultaneous direct current metal magnetron sputtering and C60 evaporation in the temperature range 200–800 °C. Thin superlattices (approximately 1000 Å) deposited at 400 °C exhibited an epitaxial growth with abrupt interfaces while films deposited at 200 °C showed a partial loss of epitaxy. At 800 °C roughening by surface diffusion started to degrade the superlattices and introduced a columnar microstructure. A loss of epitaxy was observed for thicker (>7000 Å) superlattice films deposited at 400 °C. The results suggest that this observation is due to difficulties in depositing epitaxial VC.
Films of epitaxial titanium carbide were grown on MgO(001) at 400 and 500 °C using a novel method based on the co-evaporation of Ti and C60. Mirrorlike, adhesive films of TiC1-x (0.2≪x≪0.4) were deposited at growth rates of approximately 0.1 μm/h. X-ray diffraction showed that the crystal orientation relationship between the film and the substrate was TiC(001)//MgO(001) and TiC[100]//MgO[100]. Transmission electron microscopy and low energy electron diffraction were also used to verify the epitaxial growth of the films. © 1997 American Vacuum Society.
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.
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.
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.
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.
The electrical resistivity of single crystals of vanadium carbide has been studied as a function of composition and temperature. Particular attention has been devoted to the resistivity of the two ordered phases of vanadium carbide, V6C5 and V8C7. Both solids have been shown to undergo first-order transformations to the disordered state at elevated temperature. The results of this investigation have been used to assess the role of carbon vacancies in determining the resistivity of the disordered solids. Vacancy scattering accounts for over 90% of the residual resistivity (resistivity at liquid-He temperatures) of the disordered solids but reduces in importance as temperature increases. The change in the resistivity of V6C5 at its critical point was found to be 5.4 ± 0.3 μΩ cm or (3.6 ± 0.2)% of the total resistivity of the disordered solid. The corresponding change found for V8C7 was 20.1 ± 1.4 μΩ cm or (14.7 ± 1.0)%. The critical temperatures for disordering in the two ordered phases were determined to be (1275 ± 8) °C for V6C5 and (1124 ± 15) °C for V8C7. The vanadium-carbon system was also shown to have a strong preference for one or the other of the ordered phases for nearly all compositions in the range 0.83≤x≤0.90, where x is the carbon-to-metal ratio.
Resonantly excited carbon and oxygen x-ray-emission spectra of gaseous carbon monoxide are presented. Emission spectra obtained with selective excitation to the π* valence orbital and to various Rydberg levels are compared to satellite-free nonresonant spectra. Screening effects caused by the excited electron, creating energy shifts and intensity variations in the resonant spectra compared to the nonresonant spectra, are observed, as well as an angular dependence of the resonantly excited spectra. The experimental spectra are compared to simulated spectra where the vibronic part is computed by means of a lifetime-vibrational interference formalism. The electronic intensities are analyzed by a separate-state, self-consistent-field method and a formalism for resonant inelastic x-ray scattering, focusing on screening and angular dependence.
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.
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.
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.
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.
The formation of ternary compounds within the Ti-Al-C system was studied by magnetron sputtering for thin-film deposition and first-principles calculations for phase stability. As-deposited films were characterized with X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). The hardness and Young's moduli of the material were studied by nanoindentation. Epitaxial and phase-pure films of M(n+1)AX(n) phases Ti3AlC2 and Ti2AlC as well as the perovskite phase Ti3AlC were deposited on Al2O3(00l) wafers kept at temperatures between 800 and 900 degrees C. The only ternary phases observed at low temperatures (300 degrees C) were Ti3AlC and cubic (Ti,Al)C, the latter can be described as a metastable solid solution of Al in TiC similar to the more studied (Ti,Al)N system. The difficulties to form MAX phases at low substrate temperatures were attributed of requirement for a sufficient diffusivity to partition the elements corresponding to the relatively complex crystal structures with long c-axes. While MAX-phase synthesis at 800 degrees C is significantly lower than contemporary bulk sintering processes, a reduction of the substrate temperature towards 300 degrees C in the present thin-film deposition experiments resulted in stacking sequence variations and the intergrowth of (Ti,AI)C.
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].
The essential features of a full potential electronic structure method using Linear Muffin-Tin Orbitals (LMTOs) are presented.
The electron density and potential in the this method are represented with no inherent geometrical approximation. This method
allows the calculation of total energies and forces with arbitrary accuracy while sacrificing much of the efficiency and physical
content of approximate methods such as the LMTO-ASA method.
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 5×1012 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.
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.
The electronic structures of MC compounds (M=3d-transition-metal) in NaCl structure have been studied by using the density functional theory and the calculated results indicate that the arrangements of the band structures and total density of states of MC compounds are similar. However, the chemical bondings of these compounds are different. According to the components of the energy bands crossed by EF, the MC compounds can be approximately divided into three groups, ScC, TiC and the other MC compounds, respectively. By using two-sublattice model, the bonding energies of M–C, M–M and C–C bonds for MC compounds in NaCl structure are determined quantitatively. The results show that M–C bond has the largest contribution for the cohesive energy of each MC compound and the TiC and VC have the strongest M–C bond, and M–M bond, respectively. For those metastable MC compounds, the interaction between carbon atoms cannot be neglected. Furthermore, our calculated cohesive energies of MC compounds in NaCl structure are in agreement with the experiments.
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.
The electronic structure of nanolaminate Ti2AlN and TiN thin films has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Ti L, N K, Al L1 and Al L2,3 emission spectra are compared with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Three different types of bond regions are identified; a relatively weak Ti 3d - Al 3p bonding between -1 and -2 eV below the Fermi level, and Ti 3d - N 2p and Ti 3d - N 2s bonding which are deeper in energy observed at -4.8 eV and -15 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states of Al L2,3 emission from Ti2AlN in comparison to pure Al metal is found, which reflects the Ti 3d - Al 3p hybridization observed in the Al L1 emission. The differences between the electronic and crystal structures of Ti2AlN and TiN are discussed in relation to the intercalated Al layers of the former compound and the change of the materials properties in comparison to the isostructural carbides.
The L2,3 X-ray emission of Cu metal has been measured using monochromatic
synchrotron radiation. The self-absorption effect in the spectra is shown to be
very small in our experimental geometry. From the quantitative analysis of
spectra recorded at different excitation energies, the L3/L2 emission intensity
ratio and the partial Auger-width are extracted. High-energy satellite features
on the L3 emission line are separated by a subtraction procedure. The satellite
intensity is found to be slowly increasing for excitation energies between the
L3, L2 and L1 core-level thresholds due to shake-up and shake-off transitions.
As the excitation energy passes the L2 threshold, a step of rapidly increasing
satellite intensity of the L3 emission is found due to additional Coster-Kronig
processes.
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.
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.
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\|.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
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