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Structure and Bonding in Amorphous Cr 1-x C x Nanocomposite Thin Films: X-ray Absorption Spectra and First-Principles Calculations
Abstract
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.
... From Fig. 6(a), it is noticeable that the Cr K edge XANES spectra of Cr 2 O 3 deposited on bare WC-Co substrate and pre-etched substrate are almost the same. As reported in other literatures [30][31][32][33], the feature of Cr 2 O 3 spectrum is the double peaks in the near-edge region and the shoulder at approximately 6000 eV. Combining the XANES spectra of Cr 2 O 3 with the XRD pattern shown in Fig. 5, it is reasonable to conclude that the Cr 2 O 3 interlayer keeps the same chemical state after diamond deposition. ...
... The position of the pre-edge peak depends on the chemical state of the absorbing element. In the case of chromium carbide, the pre-edge peak is caused by the transition of electron from 1 s to 3d and admixed with carbon's p orbital to form hybridized Cr3d-C2p states [33,34]. From the result reported by Olovsson, the edge energy E 0 increases with the carbon concentration inside the chromium carbide [33]. ...
... In the case of chromium carbide, the pre-edge peak is caused by the transition of electron from 1 s to 3d and admixed with carbon's p orbital to form hybridized Cr3d-C2p states [33,34]. From the result reported by Olovsson, the edge energy E 0 increases with the carbon concentration inside the chromium carbide [33]. The upward shift of E 0 means the higher ionization of the chemical state of the absorbing Cr atoms when carbon fraction increases [33]. ...
Cr2O3-Cr interlayer was prepared by magnetron sputtering to restrict graphite formation on WC-Co before diamond nucleation. Continuous diamond coatings containing little graphite phase were successfully deposited on single Cr2O3 interlayered WC-Co substrates. For Cr2O3/Cr duplex interlayer, a low nucleation density was observed at a low CH4 concentration, but a continuous diamond coating was readily obtained with an increased CH4 concentration during nucleation stage. X-ray diffraction and X-ray absorption analysis reveal that the Cr top layer was carburized to Cr3C2 and Cr7C3 after diamond deposition while Cr2O3 layer kept unchanged. In addition, the indentation test shows that the diamond coating has poor adhesion to the substrate with a single Cr2O3 interlayer, while the adhesion was significantly enhanced with the Cr2O3/Cr duplex interlayer combined with a high CH4 concentration for nucleation.
... Both near-edge XANES and EXAFS with an extended energy regime contain information about the local geometry near the excited atoms and the methods complement each other. Although the scattering cross section for hydrogen is relatively low, these spectroscopies are ideal tools for characterizing the local short-range atomic coordination symmetry 19 and chemical bonding in both crystalline and amorphous materials 20,21 . XANES provides quantitative information about the average oxidation state of the absorbing element, the local coordination environment and the unoccupied electronic structure while EXAFS provides quantitative information about average bond length, the coordination numbers and type of neighbors as well as the mean-square disorder of the nearest neighboring atoms 22,23 . ...
... Above 3.5 Å, smaller peaks are observed due to the second and higher coordination shells. 21 For x=0.15, (the ZrH 0.15 film in Figure 7), we observe a small positive shift to slightly larger distance (+0.02 Å) corresponding to an expansion of the lattice. ...
The variation in local atomic structure and chemical bonding of ZrHx (x=0.15, 0.30, 1.16) magnetron sputtered thin films are investigated by Zr K-edge (1s) X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies. A chemical shift of the Zr K-edge towards higher energy with increasing hydrogen content is observed due to charge-transfer and an ionic or polar covalent bonding component between the Zr 4d and the H 1s states with increasing valency for Zr. We find an increase in the Zr-Zr bond distance with increasing hydrogen content from 3.160 {\AA} in the hexagonal closest-packed metal (alpha-phase) to 3.395 {\AA} in the understoichiometric delta-ZrHx film (CaF2-type structure) with x=1.16 that largely resembles that of bulk delta-ZrH2. For yet lower hydrogen contents, the structures are mixed alpha and delta-phases, while sufficient hydrogen loading (x>1) yields a pure {\delta}-phase that is understoichiometric, but thermodynamically stable. The change in the hydrogen content and strain is discussed in relation to the corresponding change of bond lengths, hybridizations, and trends in electrical resistivity.
... Experimentally, oxidation states and local short-range order atomic coordination symmetry have been probed with the combination of x-ray absorption near-edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy [132] that are complementary tools to long-range order probed by x-ray diffraction (XRD). XANES provides; (i) a quantitative measure of the average oxidation state by the energy shift of the appropriate x-ray absorption edge; (ii) the amount of p-d hybridization in the chemical bonds, and thus the symmetry of the structure; and (iii) information about the coordination symmetry when the XANES line shape is compared to reference materials [133]. ...
... This was also demonstrated in nanostructured Zr1-xTaxBy core/shell thin films [213]. These core/shell nanostructured films combined the properties of crystalline diboride nanocolumns, providing high hardness, while Ta-rich dense shells were disordered (and prefer a solid solution) similar to metallic glasses [132] [216] that give rise to increased toughness. The mechanical properties are further discussed in section 5.3. ...
We review the thin film growth, chemistry, and physical properties of Group 4-6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... Experimentally, oxidation states and local short-range order atomic coordination symmetry have been probed with the combination of x-ray absorption near-edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy [132] that are complementary tools to long-range order probed by x-ray diffraction (XRD). XANES provides; (i) a quantitative measure of the average oxidation state by the energy shift of the appropriate x-ray absorption edge; (ii) the amount of p-d hybridization in the chemical bonds, and thus the symmetry of the structure; and (iii) information about the coordination symmetry when the XANES line shape is compared to reference materials [133]. ...
... This was also demonstrated in nanostructured Zr 1-x Ta x B y core/shell thin films [213]. These core/shell nanostructured films combined the properties of crystalline diboride nanocolumns, providing high hardness, while Ta-rich dense shells were disordered (and prefer a solid solution) similar to metallic glasses [132,216] that give rise to increased toughness. The mechanical properties are further discussed in section 5.3. ...
We review the thin film growth, chemistry, and physical properties of Group 4–6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... In the field of protective coatings, a prime example of a-C based nanocomposite coatings is Me-C/a-C system (Me = Ti [6,9], Zr [10], Nb [11] and Cr [12,13]) that goes to the second kind of nanocomposite structures in previous description. Among the literatures related to CreC system, only a few works have clearly identified the presence of nanocomposite structures. ...
... Singh et al. synthesized a kind of nanocomposite structure by a hybrid PVD/PECBVD process in which chromium carbide nanoclusters are embedded in amorphous carbon matrix at a low Cr content (1.5 at.%), exhibiting excellent tribological properties. Olovsson et al. [12] prepared amorphous two phase Cr 1−x C x nanocomposite thin films using direct current magnetron sputtering, and corresponding local structure and chemical bonding were discussed. In addition, a survey of literature shows that reactive deposition tends to give more crystalline coatings when hydrocarbons are used as the carbon source [15][16][17]. ...
Cr–C nanocomposite coatings were deposited by multi-arc ion plating with various argon/acetylene mixtures. The XRD, XPS and HRTEM characterization suggest that the coatings consist of a carbide phase and an amorphous carbon-rich phase. The carbide phase is shown to be CrC with a fcc NaCl (B1) structure. Experiment findings and theoretical calculations reveal phase segregation of sp²-carbon from carbides in which the excess amounts of carbon are incorporated. Nanoindentation, scratch test and Rockwell indentation were performed to investigate the toughness and adhesion ability. A significant improvement of adhesion and toughness is achieved by increasing C content at the expense of hardness. The wear resistance of Cr–C coatings is controlled by the friction rather than hardness. Compared to dry friction, lower friction coefficients but higher wear rates were obtained under seawater condition. The corrosion behavior of Cr–C nanocomposite coatings with different C % was determined by potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS). The coatings with higher Cr content display excellent anti-corrosion properties in seawater.
... When probing the pre-edge and nearedge absorption structures, the technique is also referred to as X-ray Absorption Near-Edge Structure (XANES) for solids or Near Edge X-ray Absorption Fine Structure (NEXAFS) for surfaces [32] while the long-range post-edge oscillations are used for structural determinations using Extended X-ray Absorption Fine Structure (EXAFS) [33]. As the XAS technique requires an intense (and often polarized) X-ray beam in a range of photon energies in the vicinity of a corelevel energy (1s, 2p, 3p-shells) that depends on the element of interest, XAS is nowadays performed at synchrotron radiation sources by scanning the photon energy over the absorption edge [34]. The measurements can be performed either in surface-sensitive Total Electron-Yield (TEY) mode or in a more bulk-sensitive Total Fluorescence Yield (TFY) mode [22] at different incidence angles. ...
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 exhibit 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 materials properties makes it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The bottom part of Fig. 3 shows the first derivative of the absorption spectra presented above. The energy positions and the shapes of the main absorption peaks and the pre-edge XANES shoulders depend on the chemical state of the absorbing atom [35]. For the two ZrB2 samples, the position of the absorption edge-step has moved to higher energy due to charge-transfer from the Zr to the B atoms. ...
The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk {\alpha}-Zr reference. Quantitative analysis of X-ray Photoelectron Spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk {\alpha}-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible -111, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk {\alpha}-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the 0001-oriented metal. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to {\alpha}-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modelled cell-edge in ZrB2 is slightly smaller in the thin film (a=3.165 {\AA}, c=3.520 {\AA}) in comparison to the bulk target material (a=3.175 {\AA}, c=3.540 {\AA}) while in hexagonal closest-packed metal ({\alpha}-phase, a=3.254 {\AA}, c=5.147 {\AA}).
... An accurate and valid atomic structure is a prerequisite for meaningful first-principles modelling of other physical properties. Extended X-ray absorption fine structure (EXAFS) data are sensitive to the shortrange order and have been used to qualitatively validate simulated amorphous structures for a few amorphous alloys 9,17,18 . Many details of the atomic structure of amorphous systems are however still unknown. ...
Amorphous alloys exhibit useful properties such as the excellent soft magnetic behaviour of Fe-based metallic glasses. The detailed structure of amorphous Fe(1-x)Zrx with x = 0.07, 0.10, and 0.20 is in this work explored through a synergetic combination of atomistic simulations and experimental characterisation. Thin-film samples were investigated using X-ray diffraction and extended X-ray absorption fine structure (EXAFS), while the corresponding atomic structures were simulated using an efficient first-principles-based method called stochastic quenching (SQ). The simulated local atomic arrangements are investigated by constructing the radial- and angular-distribution functions, as well as by Voronoi tesselation. The radial distribution functions are then used to construct a model to fit simultaneously the experimental EXAFS data of multiple samples with different compositions, creating a simple yet accurate description of the atomic structures valid for any composition in the range x = 0.07 to 0.20, using a minimal number of free parameters. This approach significantly improves the accuracy of the fitted parameters and allows us to relate the compositional dependence of the amorphous structures with the magnetic properties. The proposed EXAFS fitting process can be generalised to other amorphous systems, contributing to the understanding of structure-property relationships and the development of amorphous alloys with tailored functional properties.
... However, in these experiments, amorphicity is obtained by adding silicon to the system, which may be a further source of the raised resistivity, in addition to the inhibition of band formation by disorder. Nevertheless, it is not strictly necessary to introduce additional elements, such as Si [27,48] or B [47], to drive the amorphization in metal carbides, since amorphized yet metalloid-free ternary metal carbides have been reported previously [46,[49][50][51]. In the specific case of transition metal carbide films, TEM-amorphous CrC [50] and WC-based films [49] have been synthesized and their electronic structure has been characterized experimentally. ...
High entropy materials (HEMs) are of great interest for their mechanical, chemical and electronic properties. In this paper we analyse (TaNbHfTiZr)C, a carbide type of HEM, both in crystalline and amorphous phases, using density functional theory (DFT). We find that the relaxed lattice volume of the amorphous phase is larger, while its bulk modulus is lower, than that of its crystalline counterpart. Both phases are metallic with all the transition metals contributing similarly to the density of states close to the Fermi level, with Ti and Nb giving the proportionally largest contribution of states. We confirm that despite its great structural complexity, 2 × 2 × 2 supercells are large enough for reliable simulation of the presented mechanical and electronic properties by DFT.
... However, in these experiments, amorphicity is obtained by adding silicon to the system, which may be a further source of the raised resistivity, in addition to the inhibition of band formation by disorder. Nevertheless, it is not strictly necessary to introduce additional elements, such as Si [27,48] or B [47], to drive the amorphization in metal carbides, since amorphized yet metalloid-free ternary metal carbides have been reported previously [46,[49][50][51]. In the specific case of transition metal carbide films, TEM-amorphous CrC [50] and WC-based films [49] have been synthesized and their electronic structure has been characterized experimentally. ...
High entropy materials (HEMs) are of great interest for their mechanical, chemical and electronic properties. In this paper we analyse (TaNbHfTiZr)C, a carbide type of HEM, both in crystalline and amorphous phases, using density functional theory (DFT). We find that the relaxed lattice volume of the amorphous phase is larger, while its bulk modulus is lower, than that of its crystalline counterpart. Both phases are metallic with all the transition metals contributing similarly to the density of states (DOS) close to the Fermi level, with Ti and Nb giving the proportionally largest contribution of states. We confirm that despite its great structural complexity, 2x2x2 supercells are large enough for reliable simulation of the presented mechanical and electronic properties by DFT.
... Since the film content hardly changes with the sputtering depth, the atomic percentage for Cr, C and Fe can be calculated at around 38.8 %, 55 % and 6.2 % respectively. The ratio of C/(Cr + Fe) indicates the compound materials produced is highly nonstoichiometric, as often reported in the carbide coatings [12,13]. Moreover, the phase constitutions were examined by XRD under two different modes. ...
In this study, a high quality chromized coating was produced on GCr15 steel substrate by vapor phase chromizing technique for sprag clutch application. The microstructure characteristics, mechanical properties and sliding friction behaviour were systematically investigated. The results show that the chromized coating consists of 1) an outmost layer of columnar Cr 7 C 3 grains with excess carbon atoms (55 at.%) and considerable amount of iron atoms (6.2 at.%); and 2) an inner layer of coarse Fe 3 C grains. The carbide phase formation can be well explained via the thermodynamics evaluation for the Fe-Cr-C system, while the origin of carbon contamination is likely originated from the side reactions of precursor gases. The high surface hardness (16.6 GPa) and interfacial adhesion (~50 N) endow the chromized coatings with superior wear resistance against steel counterface under ambient conditions. The wear rates are found to decrease with the applied load (10-25 N). Scanning electron microscopy (SEM) analysis of the worn surfaces indicates that a chromium oxide tribolayer tends to be formed under a higher loading which displays lubrication and hence protects the contacting material during the friction process.
... 24 The problem of the self-absorption effect for XANES is usually associated with higher doping concentration samples. 25 The self-absorption effect for the XANES spectrum is also crosschecked with the Fluo algorithm in ATHENA, which has not shown any contrast between experimental and corrected self-absorption spectra. 26,27 In this study, the Ti doping concentration is very low, up to 4%. ...
In this article, we predominantly report the investigation of local crystal structure around Ti dopant by X-ray absorption spectroscopy (XAS) and nano-mechanical properties of co-sputtered Al1-xTixN (x = 0 to 4%) thin films. Grazing incidence X-ray diffraction (GIXRD) result reveals that these films are crystallized with wurtzite hexagonal structure of AlN. Surface chemical analysis and morphology of Al1-xTixN films are executed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) technique, respectively. X-ray absorption near-edge structure (XANES) exhibits that Ti atom replaces Al in AlN crystal and forms localized distorted tetrahedron TiN species, leading to a tensile strain. The bond lengths (Ti-N)ax and (Ti-N)bs are found to be moderately decreased with Ti concentration, which are extracted from the extended X-ray absorption fine structure (EXAFS) analysis. However, Ti-Al bond length in second coordination sphere is unaffected with Ti concentration having Al vacancies. The hardness (H) and modulus (E) of Al1-xTixN films are measured by nano-indentation technique, which are increased from 17.5 to 27.6 GPa and 231 to 293 GPa, respectively with x = 0 to 4 at% because of the improvement of p-d hybridization between the Ti and N atoms.
... This decrease resulted in a peak energy for the white line of ∼6011 eV which is 1 eV lower than that of the reference Cr 2 O 3 at ∼6012 eV. When features of the experimental spectra are softer or smoother than that of the calculated curves, it can reflect larger amounts of nano-or amorphous constituents in the material 31 . The nano-or amorphous constituents could be revealed by XANES but not by GIXRD since amorphous characteristic features such as peak broadening were not clearly observed in the GIXRD spectra shown in Fig. 1. ...
Martensitic stainless steel parts used in carbonaceous atmosphere at high temperature are subject to corrosion which results in a large amount of lost energy and high repair and maintenance costs. This work therefore proposes a model for surface development and corrosion mechanism as a solution to reduce corrosion costs. The morphology, phase, and corrosion behavior of steel are investigated using GIXRD, XANES, and EIS. The results show formation of nanograin-boundary networks in the protective layer of martensitic stainless steel. This Cr2O3-Cr7C3 nanograin mixture on the FeCr2O4 layer causes ion transport which is the main reason for the corrosion reaction during carburizing of the steel. The results reveal the rate determining steps in the corrosion mechanism during carburizing of steel. These steps are the diffusion of uncharged active gases in the stagnant-gas layer over the steel surface followed by the conversion of C into C4- and O into O2- at the gas-oxide interface simultaneously with the migration of Cr3+ from the metal-oxide interface to the gas-oxide interface. It is proposed that previous research on Al2O3 coatings may be the solution to producing effective coatings that overcome the corrosion challenges discussed in this work.
... The bottom part of Fig. 3 shows the first derivative of the absorption spectra presented above. The energy positions and the shapes of the main absorption peaks and the pre-edge XANES shoulders depend on the chemical state of the absorbing atom [35]. For the two ZrB2 samples, the position of the absorption edge-step has moved to higher energy due to charge-transfer from the Zr to the B atoms. ...
The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk {\alpha}-Zr reference. Quantitative analysis of X-ray Photoelectron Spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk {\alpha}-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible -111, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk {\alpha}-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the 0001-oriented metal. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to {\alpha}-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modelled cell-edge in ZrB2 is slightly smaller in the thin film (a=3.165 {\AA}, c=3.520 {\AA}) in comparison to the bulk target material (a=3.175 {\AA}, c=3.540 {\AA}) while in hexagonal closest-packed metal ({\alpha}-phase, a=3.254 {\AA}, c=5.147 {\AA}).
... Above 3.5 Å, smaller peaks are observed due to the second and higher coordination shells. 21 For x = 0.15 (the ZrH 0.15 film in Figure 7), we observe a small positive shift to slightly larger distance (+0.02 Å) corresponding to an expansion of the lattice. ...
The variation in local atomic structure and chemical bonding of ZrHx (x=0.15, 0.30, 1.16) magnetron sputtered thin films are investigated by Zr K-edge (1s) X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies. A chemical shift of the Zr K-edge towards higher energy with increasing hydrogen content is observed due to charge-transfer and an ionic or polar covalent bonding component between the Zr 4d and the H 1s states with increasing valency for Zr. We find an increase in the Zr-Zr bond distance with increasing hydrogen content from 3.160 Å in the hexagonal closest-packed metal (α-phase) to 3.395 Å in the understoichiometric -ZrHx film (CaF2-type structure) with x=1.16 that largely resembles that of bulk δ-ZrH2. For yet lower hydrogen contents, the structures are mixed α and δ-phases, while sufficient hydrogen loading (x>1) yields a pure δ-phase that is understoichiometric, but thermodynamically stable. The change in the hydrogen content and strain is discussed in relation to the corresponding change of bond lengths, hybridizations, and trends in electrical resistivity.
... When probing the pre-edge and near-edge absorption structures, the technique is also referred to as X-ray Absorption Near-Edge Structure (XANES) for solids or Near Edge X-ray Absorption Fine Structure (NEXAFS) for surfaces [32] while the long-range post-edge oscillations are used for structural determinations using Extended X-ray Absorption Fine Structure (EXAFS) [33]. As the XAS technique requires an intense (and often polarized) X-ray beam in a range of photon energies in the vicinity of a core-level energy (1s, 2p, 3p-shells) that depends on the element of interest, XAS is nowadays performed at synchrotron radiation sources by scanning the photon energy over the absorption edge [34]. The measurements can be performed either in surface-sensitive Total Electron-Yield (TEY) mode or in a more bulk-sensitive Total Fluorescence Yield (TFY) mode [22] at different incidence angles. ...
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.
... When probing the pre-edge and near-edge absorption structures, the technique is also referred to as X-ray Absorption Near-Edge Structure (XANES) for solids or Near Edge X-ray Absorption Fine Structure (NEXAFS) for surfaces [32] while the long-range post-edge oscillations are used for structural determinations using Extended X-ray Absorption Fine Structure (EXAFS) [33]. As the XAS technique requires an intense (and often polarized) X-ray beam in a range of photon energies in the vicinity of a core-level energy (1s, 2p, 3p-shells) that depends on the element of interest, XAS is nowadays performed at synchrotron radiation sources by scanning the photon energy over the absorption edge [34]. The measurements can be performed either in surface-sensitive Total Electron-Yield (TEY) mode or in a more bulk-sensitive Total Fluorescence Yield (TFY) mode [22] at different incidence angles. ...
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 optical performance of low-bilayer-thickness metallic multilayers (ML) can be improved significantly by limiting the intermixing of consecutive layers at the interfaces. Barrier layers are supposed to exhibit a decisive role in controlling diffusion across the interfaces. The element-specific grazing incidence extended X-ray absorption fine structure technique using synchrotron radiation has been used in conjunction with grazing incidence X-ray reflectivity and diffuse X-ray scattering measurements to study the impact of the two most common barrier layers, viz., C and B4C, at the interfaces of Cr/Sc MLs. The diffusion propagation is reduced by both the barrier layers; however, it is found that the improvement is more significant with the B4C barrier layer. It is seen that C forms an intermixed layer with Sc and leads to carbide formation at the interface, which then acts as shielding and prevents further interdiffusion, while B4C hardly penetrates into Sc and stops the overlap between Sc and Cr directly by wetting the corresponding interface. Thus, the above measurements reveal crucial and precise information regarding the elemental diffusion kinetics at the interfaces of Cr/Sc MLs in a non-destructive way, which is very important for technological applications of these MLs as X-ray optical devices.
CrCx coatings (where x = 5 ~ 35 sccm is the acetylene flux) were deposited on stainless steel 316L substrates using radio frequency unbalanced magnetron sputtering. The CrC35 coating showed the highest C-sp2 content and H/E ratio of all the coatings, and thus exhibited the lowest friction coefficient and wear rate. However, the CrC25 coating showed the best anti-corrosion performance in 0.5 M H2SO4 solution due to its low surface roughness and small cluster size. The electrochemical properties of the CrC25 coating were investigated in a simulated Proton Exchange Membrane Fuel Cell (PEMFC) environment with hydrogen and oxygen purging. The coating showed a low corrosion current density in both the anode and the cathode environments. Finally, the CrC35 coating showed the lowest interfacial contact resistance (ICR) of the various coatings under a compaction force of 140 Ncm−2 due to its high C-sp2 content.
Cr-CHx coatings (where x indicates the acetylene flow rate and has a value in the range of 5 to 35 sccm) were deposited on high-speed steel substrates using a radio frequency magnetron sputtering system. The crystalline phases of the deposited coatings were characterized using x-ray diffraction. The surface morphologies and cross-sectional microstructures were analyzed using scanning electron microscopy. The XRD results showed that the coatings consisted mainly of Cr7C3 and metal Cr crystalline phases. In addition, the Raman spectra analysis results showed that the Cr-CH25 and Cr-CH35 coatings had ID–IG ratios of 1.08 and 1.22, respectively. The coating hardness gradually decreased with an increasing acetylene flow rate. By contrast, the adhesion strength increased as the flow rate increased beyond 15 sccm. Among all the coatings, the Cr-CH35 coating showed the best tribological properties, with an average friction coefficient of 0.048 and wear rate of 0.28×10−6 mm3/Nm under a load of 8 N.
Electrochemical water splitting provides a promising approach to storing renewable electricity in the form of hydrogen on a grand scale. However, the current techniques for large‐scale electrochemical hydrogen production rely on the use of noble‐metal catalysts making it uncompetitive to traditional methods using fossil fuels, due to the high cost and scarcity of noble‐metal catalysts. Thus, replacing noble‐metal electrocatalysts with cheap materials made of abundant elements holds the key to achieve cost‐effectiveness. Recently, amorphous electrocatalysts emerged as promising candidates due to their unique physical and chemical properties compared to their crystalline counterparts leading to superior catalytic performance. Given the rapid advances made in the design, synthesis and development of amorphous catalysts, namely monometallic and multimetallic borides, sulfides, phosphides, oxides, and hydroxides based on transition metals, this review aims to summarize the recent progress on compositional designs, microstructure, morphology, electronic properties, and interaction with host materials. Special attention is paid to uncover the main strategies adopted in each material category and the underlying structure–property relationship that improves hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances. As a result, this review provides guidance for the design and synthesis of novel amorphous electrocatalysts with high performance for large‐scale electrochemical water splitting application.
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In order to achieve hard, tough and super wear resistant film, Ta1-xWxCy films with various W contents were deposited by DC magnetron sputtering and their structural evolution and mechanical properties were investigated. Ta1-xWxCy films show the structures of fcc-NaCl solid solution and nanocomposite, which depended on the W contents. Doping W atoms induces the amorphous carbon (a-C) precipitated at the grain boundary. The structure of Ta1-xWxCy film changes from the single solid solution to a nanocomposite structure, which consists of Ta(W)C nanocrystalline with C vacancies and small amount of a-C. Ta0.69W0.31C0.75 film shows superior mechanical properties with ultra-high hardness of 43.9 GPa and enhanced toughness (KIC) of 3.95 MPa·m1/2, which is attributed to the synergistic effect of solid solution strengthening, composite structure strengthening and toughening related to the interaction of valence electrons. Both low average friction coefficient of 0.28 and wear rate of 1.12 × 10⁻⁶ mm³/N·m is obtained from the Ta0.69W0.31C0.75 film, which is consistent well with its super mechanical properties. In this study, the fracture toughness is not directional related to H/E and H³/E² for the nanocomposite coating.
In this study, the TaCx composite films consisted of amorphous carbon and nanocrystalline TaC were synthesized with varying sputtering powers of graphite by DC magnetron sputtering, and studied their microstructure, mechanical and tribological behaviors. The film exhibits a Ta2C phase and high content a-C composite structure at a graphite power of 30 W, then a new composite structure with a lower a-C content and TaC nanocrystalline was formed, as increasing the graphite power, their hardness and toughness was increased at first and then decreased. The TaC110 film obtained the highest hardness (42.8 GPa), H/E (0.12) and elastic recovery (80.5%), this is due to the hard TaCx grains with high carbon vacancy are encompassed by suitable amorphous carbon phase, that the formed strengthening TaCx phase and nanocomposite structure greatly enhanced the mechanical properties. Nevertheless, the TaC50 film with a relatively low hardness and toughness exhibits the lowest average COF of 0.18 and wear rate of 1.44 × 10⁻⁶ mm³/N·m, which is attributed to the lubrications of partial tantalum oxide oxides and highly graphitized transfer layer on the frictional interface.
First-principles calculations are employed to determine the mechanical properties of rock-salt structure binary and ternary transition metal nitrides, carbides, and carbonitrides from groups 4 to 12, predicting a unified indicator for mechanical properties: the valence electron concentration (VEC). Pugh's and Poisson's ratios indicate an increasing ductility with increasing VEC, with a brittle-to-ductile transition at a critical VEC = 10. The calculated C44 of carbonitrides and ternary nitrides monotonically decreases from 164 ± 12 GPa at VEC = 8 to −39 ± 46 GPa at VEC = 11, indicating a transition to mechanical instability at VEC = 10.6. Similarly, the average isotropic elastic modulus decreases slightly from 420 GPa for VEC = 8 to 388 GPa for VEC = 10, but then steeply to −98 GPa for VEC = 11, while the corresponding hardness decreases from 25 to 12 to 2 GPa. The overall softening with increasing VEC is attributed to the increasing electron density in d–t2g orbitals, which overlap upon shear and cause a decrease in C44. Phonon dispersion curves, calculated at 0 K for binary nitrides and carbides, exhibit imaginary frequencies for VEC ≥10, indicating a dynamical stability-to-instability transition between VEC = 9 and 10, which is smaller than the critical VEC = 10.6 for the mechanical stability-instability transition. In addition, mechanical stability is increased by magnetic ordering but decreased when accounting for on-site Coulomb repulsion, while temperature and vacancies cause a reduction in the magnitude of C44 for both stable and unstable compounds, likely leading to an increase in the critical VEC for the stability-instability transition. The overall results indicate a narrow region between VEC = 9 and 10 where rocksalt carbonitrides are ductile but also exhibit a high hardness, mechanical and dynamical stability, and therefore are expected to exhibit the highest toughness.
The anharmonic contributions and pressure effects on extended X-ray absorption fine structure (EXAFS) Debye-Waller factor of platinum metal have been investigated up to 900 K and 14 GPa within the anharmonic correlated Debye model. Parameters of interatomic potential have been derived under the second-moment approximation of tight binding scheme. Our calculations of the EXAFS Debye-Waller factor and anharmonic effective potential are compared with those of experiments showing the good and reasonable agreements. We have shown in detail that the anharmonicity of the thermal vibration of atoms give an important contribution to EXAFS Debye-Waller factor at high temperature. And the increasing of pressure will depress the EXAFS amplitude through the reduction of atomic mean-square relative displacement characterizing the Debye-Waller factor.
The anharmonic effects of gold in extended X-ray absorption fine structure (EXAFS) have been investigated through the consideration of the first four EXAFS cumulants up to temperature 800 K within the anharmonic correlated Debye model. The interatomic potential between two intermediate atoms has been described by the second-moment approximation to the tight-binding model and its parameters were determined from first-principles calculations. Our results of the first four EXAFS cumulants and anharmonic effective potential are compared with those of experiments showing the good and reasonable agreements. We have shown in detail that the anharmonicity contributions of the thermal vibration of atoms are important to EXAFS cumulants at high temperature.
We report how the total carbon content and the amorphous carbon (a-C) phase fraction in transition metal carbide/a-C nanocomposite coatings can be obtained using optical methods, which are much more practical for industrial use than conventional X-ray photoelectron spectroscopy. A large set of carbon-containing nanocomposite coatings deposited using different magnetron sputtering techniques were analyzed by X-ray photoelectron spectroscopy, reflectance spectrophotometry, and spectroscopic ellipsometry. The chemical composition and the a-C phase fraction were determined by X-ray photoelectron spectroscopy for each coating and results are presented for the TiC, CrC, and NbC systems. The composition and the a-C phase fraction are correlated to optical reflectance in the visible range, by parametrization in L*a*b* color space, and by ellipsometry primary data. Results show that it is possible to rapidly estimate the composition and the a-C fraction using these optical methods. We propose that optical methods have promising use in the industry as a cost-efficient technique for characterization of carbide-based coatings.
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 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.
We have carried out a theoretical and experimental investigation of the beryllium K-edge soft x-ray absorption fine structure of beryllium compounds in the oxygen group, considering BeO, BeS, BeSe, and BeTe. Theoretical spectra are obtained ab initio, through many-body perturbation theory, by solving the Bethe-Salpeter equation (BSE), and by supercell calculations using the core-hole approximation. All calculations are performed with the full-potential linearized augmented plane-wave method. It is found that the two different theoretical approaches produce a similar fine structure, in good agreement with the experimental data. Using the BSE results, we interpret the spectra, distinguishing between bound core-excitons and higher energy excitations.
Dramatic advances in the understanding of x-ray absorption fine structure (XAFS) have been made over the past few decades, which have led ultimately to a highly quantitative theory. This review covers these developments from a unified multiple-scattering viewpoint. The authors focus on extended x-ray absorption fine structure (EXAFS) well above an x-ray edge, and, to a lesser extent, on x-ray absorption near-edge structure (XANES) closer to an edge. The discussion includes both formal considerations, derived from a many-electron formulation, and practical computational methods based on independent-electron models, with many-body effects lumped into various inelastic losses and energy shifts. The main conceptual issues in XAFS theory are identified and their relative importance is assessed; these include the convergence of the multiple-scattering expansion, curved-wave effects, the scattering potential, inelastic losses, self-energy shifts, and vibrations and structural disorder. The advantages and limitations of current computational approaches are addressed, with particular regard to quantitative experimental comparisons.
We present a first principles supercell methodology for the calculation of
exchange interactions of magnetic materials with arbitrary degrees of
structural and chemical disorder in their high temperature paramagnetic state.
It is based on a projection of the total magnetic energy of the system onto
local pair clusters, allowing the interactions to vary independently as a
response to their local environments. We demonstrate our method by deriving the
distance dependent exchange interactions in vibrating crystalline CrN, a
TiCrN solid solution as well as in amorphous CrN. Our method
reveals strong local environment effects in all three systems. In the amorphous
case we use the full set of exchange interactions in a search for the
non-collinear magnetic ground state.
We investigate the distribution of local minima in the potential-energy landscape of metals. The density of energy minima is calculated for Na by using a pair-potential method to quench from stochastic configurations for system sizes ranging from 1 to 4000 atoms. We find a minimum system size, approximately 150 atoms, above which the density of energy minima is dominated by one sharp peak. As the system size is increased, the peak position converges to an asymptotic value and its width converges to zero. The findings of the pair-potential method for Na are confirmed by first-principles calculations of amorphous Al and V. Finally we present an example in which our results are applied to the complex bulk metallic glass Zr52.5Cu17.9Ni14.6Al10Ti5 (Vitreloy 105). The calculated density and bulk modulus of the Vitreloy are in good agreement with experiments. The analysis presented here shows that the thermodynamic limit is better described by one large supercell calculation than by an average over many smaller supercell calculations. We argue that the minimum cell size that is needed to accurately perform such a large supercell calculation for metallic glasses is about 150 atoms.
We investigate core excitations of the Al 2p edge in the III-V semiconductors AlP, AlAs, AlSb, and AlN. For the latter, we consider the wurtzite, zinc-blende, and rock-salt polymorphs. First-principles calculations are performed utilizing two different approaches, which are the solution of the Bethe-Salpeter equation (BSE) as well as the supercell technique employing the core-hole approximation. In addition, measurements of the electron energy-loss near-edge structure of the metastable AlN phase are presented. We find that the relative intensities of the spectral features are better described by the BSE than by the supercell method. We analyze the character of the near-edge peaks and trace back their origin to strongly bound core excitons in the case of AlSb and rock-salt AlN.
We propose two methods for obtaining the atomic-like background
for the x-ray absorption fine structure (XAFS): the methods of smoothing
spline and of Bayesian smoothing. Both are capable of using the prior
information, calculated or experimental, about the background. The XAFS
signals obtained by these techniques are shown to be significantly
corrected in comparison with standard methods. The method of Bayesian
smoothing is the only method that gives the errors of approximation of the
atomic-like background by an artificial smooth function. These errors are
shown to be the main source of the uncertainty of the XAFS function.
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
It is possible in principle to probe the many-atom potential surface using density functional theory (DFT). This will allow us to apply DFT to the Hamiltonian formulation of atomic motion in monatomic liquids by Wallace [Phys. Rev. E 56, 4179 (1997)]. For a monatomic system, analysis of the potential surface is facilitated by the random and symmetric classification of potential-energy valleys. Since the random valleys are numerically dominant and uniform in their macroscopic potential properties, only a few quenches are necessary to establish these properties. Here we describe an efficient technique for doing this. Quenches are done from easily generated "stochastic" configurations, in which the nuclei are distributed uniformly within a constraint limiting the closeness of approach. For metallic Na with atomic pair potential interactions, it is shown that quenches from stochastic configurations and quenches from equilibrium liquid molecular dynamics configurations produce statistically identical distributions of the structural potential energy. Again for metallic Na, it is shown that DFT quenches from stochastic configurations provide the parameters which calibrate the Hamiltonian. A statistical mechanical analysis shows how the underlying potential properties can be extracted from the distributions found in quenches from stochastic configurations.
The characteristics of pre-edge peaks in K-edge x-ray absorption near edge structure (XANES) spectra of 3d transition metals were reviewed from viewpoints of the selection rule, coordination number, number of d-electrons, and symmetry of the coordination sphere. The contribution of the electric dipole and quadrupole transition to the peaks was discussed on the basis of the group theory, polarized spectra, and theoretical calculations. The pre-edge peak intensity for Td symmetry is larger than those for Oh symmetry for all 3d elements. The intense pre-edge peak for tetrahedral species of 3d transition metals is not due to 1s–3d transition, but transition to the p component in d–p hybridized orbital. The mixing of metal 4p orbitals with the 3d orbitals depends strongly on the coordination symmetry, and the possibility is predictable by group theory. The transition of 1s electron to d orbitals is electric quadrupole component in any of the symmetries. The d–p hybridization does not occur with regular octahedral symmetry, and the weak pre-edge peak consists of 1s–3d electric quadrupole transition. The pre-edge peak intensity for a compound with a tetrahedral center changes as a function of the number of 3d electrons regardless of the kind of element; it is maximized at d0 and gradually decreases to zero at d10. The features of pre-edge peaks in K-edge XANES spectra for 4d elements and the L1-edge for 5d elements are analogous with those for 3d elements, but the pre-edge peak is broadened due to the wide natural width of the core level. Copyright © 2008 John Wiley & Sons, Ltd.
Calcium chromates with the empirical formulas Ca 10Cr 6-O 25, Ca 3Cr 2O 8, and Ca 5Cr 3O 12, which form at temperatures >900°C at CaO:Cr 2O 3 molar ratios of ≥3 in an oxidation atmosphere have been synthesized in the pure state. X-ray absorption near-edge structure (XANES) spectroscopy has been used to determine the average valence state of chromium in the samples. The presence of unusual chromium valence states, 4+ and 5+, which was proposed via X-ray diffractometry studies, is strongly supported.
VESTA
is a three-dimensional visualization system for crystallographic studies and electronic state calculations. It has been upgraded to the latest version,
VESTA 3
, implementing new features including drawing the external morphology of crystals; superimposing multiple structural models, volumetric data and crystal faces; calculation of electron and nuclear densities from structure parameters; calculation of Patterson functions from structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels; determination of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex molecules and cage-like structures; undo and redo in graphical user interface operations; and significant performance improvements in rendering isosurfaces and calculating slices.
The structure of metallic glasses (MGs) has been a long-standing mystery. On the one hand, MGs are amorphous materials with no long-range structural order; on the other hand, topological and chemical short-to-medium range order is expected to be pronounced in these alloys, due to their high atomic packing density and the varying chemical affinity between the constituent elements. The unique internal structure of MGs underlies their interesting properties, which render MGs potentially useful for various applications. While more and more glass-forming alloys have been developed in recent years, fundamental knowledge on the structural aspect of MGs remains seriously lacking. For example, how atoms pack on the short-to-medium range, how the structure differs in different MGs and changes with composition, temperature, and processing history, and more importantly, how the structure influences the properties of MGs, are still unresolved questions. In this paper, we review the tremendous efforts over the past 50 years devoted to unraveling the atomic-level structure of MGs and the structural origin of their unique behaviors. Emphasis will be placed on the progress made in recent years, including advances in structural characterization and analysis of prototypical MGs, general structural models and fundamental principles, and the correlations of thermodynamic, kinetic, and mechanical properties with the MG structures. Some widely observed property–property correlations in MGs are also examined from the structural perspective. The insights summarized are shown to shed light on many intriguing behaviors of the MG-forming alloys and expected to impact the development of MGs. Outstanding questions in this important research area will also be outlined.
Thin films in the Cr–C system with carbon content of 25–85 at.% have been deposited using non-reactive DC magnetron sputtering from elemental targets. Analyses with X-ray diffraction and transmission electron microscopy confirm that the films are completely amorphous. Also, annealing experiment show that the films had not crystallized at 500 °C. Furthermore, X-ray spectroscopy and Raman spectroscopy show that the films consist of two phases, an amorphous CrCx phase and an amorphous carbon (a-C) phase. The presence of two amorphous phases is also supported by the electrochemical analysis, which shows that oxidation of both chromium and carbon contributes to the total current in the passive region. The relative amounts of these amorphous phases influence the film properties. Typically, lower carbon content with less a-C phase leads to harder films with higher Young’s modulus and lower resistivity. The results also show that both films have lower currents in the passive region compared to the uncoated 316L steel substrate. Finally, our results were compared with literature data from both reactively and non-reactively sputtered chromium carbide films. The comparison reveals that non-reactive sputtering tend to favour the formation of amorphous films and also influence e.g. the sp2/sp3 ratio of the a-C phase.
Thin films based on transition-metal carbides exhibit many interesting physical and chemical properties making them attractive for a variety of applications. The most widely used method to produce metal carbide films with specific properties at reduced deposition temperatures is sputter deposition. A large number of papers in this field have been published during the last decades, showing that large variations in structure and properties can be obtained. This review will summarise the literature on sputter-deposited carbide films based on chemical aspects of the various elements in the films. By considering the chemical affinities (primarily towards carbon) and structural preferences of different elements, it is possible to understand trends in structure of binary transition-metal carbides and the ternary materials based on these carbides. These trends in chemical affinity and structure will also directly affect the growth process during sputter deposition. A fundamental chemical perspective of the transition-metal carbides and their alloying elements is essential to obtain control of the material structure (from the atomic level), and thereby its properties and performance. This review covers a wide range of materials: binary transition-metal carbides and their nanocomposites with amorphous carbon; the effect of alloying carbide-based materials with a third element (mainly elements from groups 3 through 14); as well as the amorphous binary and ternary materials from these elements deposited under specific conditions or at certain compositional ranges. Furthermore, the review will also emphasise important aspects regarding materials characterisation which may affect the interpretation of data such as beam-induced crystallisation and sputter-damage during surface analysis.
During the last years, several binary transition metals (TM)–Carbon systems have been explored with the aim of, first obtaining amorphous alloys in a wide range of composition, especially towards the carbon-rich concentrations and second, studying the thermal stability and the crystallization of these new materials. Sputtering has been chosen as means of elaboration to obtain the films and electron probe microanalysis was used to determine the composition. The as-sputtered amorphous state was detected by electron diffraction and/or X-ray. For each amorphous film, the thermal stability was studied by differential scanning calorimetry and the crystallization was followed by hot-stage transmission electron microscopy. The products of crystallization were identified by electron diffraction. In this paper, we present a comparison of the main results we have got on amorphous and then crystallized films belonging to the well-known Fe–C, Mn–C and Cr–C systems. The thermal stability increases from Fe–C to Cr–C systems. Depending both on the carbon content and the nature of the transition metal, various unknown carbides form from the amorphous films. We find that they are often isomorphous with interstitial compounds already existing among borides, nitrides, carbonitrides and other carbides. The emphasis is put in their structural description. It is thus demonstrated that the new structures can offer either prismatic, octahedral or both sites to the C atoms. This suggests that more than one type of local orders may exist in the amorphous state for these TM–C systems.
This paper deals with the ground state of an interacting electron gas in an external potential v(r). It is proved that there exists a universal functional of the density, Fn(r), independent of v(r), such that the expression Ev(r)n(r)dr+Fn(r) has as its minimum value the correct ground-state energy associated with v(r). The functional Fn(r) is then discussed for two situations: (1) n(r)=n0+n(r), n/n01, and (2) n(r)= (r/r0) with arbitrary and r0. In both cases F can be expressed entirely in terms of the correlation energy and linear and higher order electronic polarizabilities of a uniform electron gas. This approach also sheds some light on generalized Thomas-Fermi methods and their limitations. Some new extensions of these methods are presented.
There has been dramatic progress in recent years both in the calculation and interpretation of various x-ray spectroscopies. However, current theoretical calculations often use a number of simplified models to account for many-body effects, in lieu of first principles calculations. In an effort to overcome these limitations we describe in this article a number of recent advances in theory and in theoretical codes which offer the prospect of parameter free calculations that include the dominant many-body effects. These advances are based on ab initio calculations of the dielectric and vibrational response of a system. Calculations of the dielectric function over a broad spectrum yield system dependent self-energies and mean-free paths, as well as intrinsic losses due to multi-electron excitations. Calculations of the dynamical matrix yield vibrational damping in terms of multiple-scattering Debye–Waller factors. Our ab initio methods for determining these many-body effects have led to new, improved, and broadly applicable x-ray and electron spectroscopy codes. To cite this article: J.J. Rehr et al., C. R. Physique 10 (2009).
A survey of investigations concerning 3d TM1−xCx films obtained in a large atomic carbon content x in several binary TM–C systems is presented. Sputtering was chosen as means of elaboration and electron probe microanalysis was used to determine the composition of the films. Their structural states were analyzed by X-ray and transmission electron microscopy. The glass-forming range and the glass thermal stability are described and compared. Adding carbon to the structure of TM-based films for Cr, Mn, Fe and Co causes a fully amorphization coming from the destabilization of the solid solution and extended in a large x range whereas for V and Ni, only a partly amorphization is detected in a narrow x range shifted towards higher x values. The thermal stability of the fully or partly amorphous films increases when TM moves from Ni to V. The role of the carbon–metal interaction is highlighted.
Metastable WMeC (MeFe, Co) films were obtained by non-reactive RF magnetron sputtering. Their initial state and thermal stability were studied by means of electron probe microanalysis, secondary ion mass spectrometry and X-ray diffraction. The results show that the structure at room temperature depends on the percentage of iron and cobalt in the films, varying from β-WC1−x to amorphous with the increasing content of these elements. The structure of the crystalline films changes with temperature from β-WC1−x to WC and W2C. The crystallization mechanism of the amorphous films is characterized by the formation of carbon-deficient phases, such as W2C or M6C.
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 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.
Using first-principles calculations, we systematically studied the structural, elastic, and electronic properties of the technologically important chromium carbides: Cr3C2, Cr7C3, Cr23C6, Cr3C, and CrC. Our calculations show that the ground state structure for Cr7C3 is hexagonal, not orthorhombic. We further predict WC to be the energetically most stable structure for CrC. Our results indicate that all chromium carbides considered in this study are metallic and mechanically stable under the ambient condition. Among all chromium carbides, WC-type CrC exhibits the highest bulk and shear moduli and the lowest Poisson’s ratio, and is a potential low-compressibility and hard material.
Many of the structural elements of importance in materials applications (e.g., thin films, barrier layers, intergranular films in ceramics) are small in volume and amorphous. Although the characterization of the structure of amorphous materials by X-ray and neutron diffraction methods is well established, these techniques are not suitable for studies of nanovolumes of materials because of the relatively small scattering cross sections. This chapter reviews recent developments in electron techniques, and particularly electron diffraction, for overcoming this problem.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
Calcium chromates with the empirical formulas Ca 10 Cr 6 −O 25 , Ca 3 Cr 2 O 8 , and Ca 5 Cr 3 O 12 , which form at temperatures >900°C at CaO:Cr 2 O 3 molar ratios of 3 in an oxidation atmosphere have been synthesized in the pure state. X‐ray absorption near‐edge structure (XANES) spectroscopy has been used to determine the average valence state of chromium in the samples. The presence of unusual chromium valence states, 4+ and 5+, which was proposed via X‐ray diffractometry studies, is strongly supported.
We have studied the Cr7C3 phase by means of ab initio calculations and found covalent-ionic Cr–C–Cr chains in a metallic matrix. Furthermore, the structure of thin films, synthesized by RF magnetron sputtering, is shown to be in good agreement with our theoretical prediction.
The microstructure, electronic structure and chemical bonding of chromium carbide thin films with different carbon contents have been investigated with high-resolution transmission electron microscopy, electron energy loss spectroscopy and soft x-ray absorption-emission spectroscopies. Most of the films can be described as amorphous nanocomposites with non-crystalline CrC(x) in an amorphous carbon matrix. At high carbon contents, graphene-like structures are formed in the amorphous carbon matrix. At 47 at.% carbon content, randomly oriented nanocrystallites are formed creating a complex microstructure of three components. The soft x-ray absorption-emission study shows additional peak structures exhibiting non-octahedral coordination and bonding.
The bulk chemistry of commercial carbon blacks and carbon blacks obtained by vacuum pyrolysis (CBP) of used tires was investigated by 13C-NMR spectroscopy with and without magic angle spinning of the sample. Two different kinds of carbon atoms can be distinguished: Graphite like carbon atoms in poly-condensed aromatic rings and carbon atoms in a less ordered environment. Commercial carbon blacks and CBP obtained under different pyrolysis conditions have practically the same concentrations of the different types of carbon atoms in the bulk, whereas earlier ESCA and SIMS investigations have shown that the surface chemistry of CBP is different from commercial carbon blacks and depends strongly on the pyrolysis conditions. Thus, during the pyrolysis only the carbon black surface chemistry is changed. The carbon black bulk structure was also studied by X-ray diffraction. The XRD results, including the radial distribution function (RDF) indicated, in agreement with the NMR results, that the bulk structure of commercial carbon blacks and of CBP are similar.
The structural behaviour of amorphous W46Fe13C41 and W36Fe31C33 films produced by sputtering have been studied in situ during annealing up to ≈950°C by means of hot stage transmission electron microscopy. Differential thermal analysis and X-ray diffraction were used as complementary experimental techniques. The results are presented and correlated with the equilibrium phases anticipated from the WFeC ternary phase diagram and with previous studies on similar films deposited and annealed onto substrates.
Composite Cr-containing hydrogenated amorphous diamond-like carbon (Cr-DLC) films were synthesized by a hybrid PVD/CVD plasma-assisted deposition process. In a recent study, it was found that Cr-DLC films with <∼12 at.% Cr possess excellent tribological properties. However, the role of Cr in inducing these characteristics is not clear. In the present report, the local structure around the Cr atoms in the latter films was studied as a function of Cr content by X-ray absorption spectroscopy. The Cr K-edge X-ray absorption near edge structure spectra show that Cr in DLC has a chemical state similar to that of chromium carbide. Analysis of the extended X-ray absorption fine structure spectra shows that at low Cr content (<0.4 at.% Cr), Cr is dissolved in the amorphous DLC matrix forming an atomic-scale composite. Simulation studies suggest that in the latter films, Cr tends to be present as very small atomic clusters of 2–3 Cr atoms. At higher Cr contents (>1.5 at.%), Cr is present as nanoparticles (<10 nm) of a defected carbide structure forming a nanocomposite.
The growth of chromium carbide coatings by pyrolysis of dicumene chromium in a hot-wall low pressure chemical vapour deposition reactor has been investigated between 300 and 550 °C. Amorphous chromium carbide films were obtained in the low temperature range 300 - 500 °C whereas a textured crystalline Cr7C3 phase was grown above 500 °C. The total carbon content was independent of the deposition temperature and amounted to a carbon excess of about 30% compared with the Cr7C3 stoichiometry. Electron spectroscopy for chemical analysis of both amorphous and crystalline coatings confirms the presence of this carbon excess since about 30% - 40% of free carbon was found. A heterogeneous structural model composed of the Cr7C3 phase and free carbon is proposed both for crystalline and amorphous coatings.
Diamondlike carbon (DLC) films, known for exhibiting attractive combination of properties, have been extensively studied in the recent past. The inherent, internal compressive stresses affecting their adhesion and their relatively low thermal stability above 400 °C are two major drawbacks preventing wide usage of these films. Carbide formers incorporated into the carbon network have the potential to stabilize the film structure, relax internal stresses and improve their performance. The present work focuses on the synthesis, structure and mechanical and tribological property characterization of Cr-containing nanocomposite DLC films. The films were synthesized using a plasma-enhanced hybrid chemical vapor and physical vapor deposition process in a discharge composed of a mixture of CH4 and Ar gases. The Cr content in the films varied up to 18 at.%. The film morphology and composition were characterized by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and nuclear reaction analysis. The mechanical and tribological behavior of the films was studied as a function of Cr concentration by conducting nanoindentation and pin-on-disc experiments, respectively. The results showed that the films can be either amorphous with dispersed metallic-like Cr (at low Cr content) or nanocomposite consisting of face-centered cubic metastable CrC nanoparticles dispersed in the DLC matrix. Films with low Cr content (< 5 at.%) were found to possess similar tribological characteristics with those of pure DLC films. Incorporation of more Cr (> 12 at.%) results in larger chromium carbide particles that have an adverse effect on wear resistance. The films with the low Cr content offer the opportunity to combine the excellent tribological behavior with other desirable properties deriving from the presence of the second phase.
X-ray absorption spectra at the chromium K-edge are reported for a number of selected chromium compounds of known chemical structure. The spectra were obtained with use of synchrotron radiation available at the ectron tretcher ccelerator ELSA in Bonn. The compounds studied include the tetrahedrally coordinated compounds Ca2Ge0.8Cr0.2O4, Ba2Ge0.1Cr0.9O4, Sr2CrO4, Ca2(PO4)x(CrO4)1−xCl (x=0.25,0.5), Ca5(CrO4)3Cl, CrO3, the octahedrally coordinated compounds Cr(II)-acetate, CrCl3, CrF3, Cr2O3, KCr(SO4)2 · 12H2O, CrO2 and cubic coordinated metallic chromium. In these compounds chromium exhibits a wide range of formal oxidation states (0 to VI). The absorption features in the near edge region are shown to be characteristic of the spatial environment of the absorbing atom. The occurrence of a single pre-edge line easily allows one to distinguish between tetrahedral and octahedral coordination geometry, whereas the energy position of the absorption edge is found to be very sensitive to the valency of the excited chromium atom. Calculations of the ionisation potential of Cr in different oxidation states using the non-relativistic Hartree–Fock method (Froese–Fischer) confirm that the ionisation limit shifts to higher energy with increasing Cr valency. More detailed information on the electronic structure of the different compounds is gained by real-space full multiple scattering calculations using the FEFF8 code.
A new model for the so-called transition metal—metalloid glasses has been constructed in which groups of atoms - coordination polyhedra with defined local geometry - are packed randomly to form a dense, three-dimensional array. Specifically, the coordination polyhedra are trigonal prisms containing six metallic atoms in a nearest neighbour sub-shell around a central non-metallic element with three further metal atoms at somewhat larger distances. They are essentially identical to the structural units observed in crystalline transition metal borides, carbides, silicides and phosphides. Computed density, neutron structure factor and radial distribution functions are in agreement with experimental data for a-Pd4Si. The local coordination can be distorted and changes in the properties of this structure provide some insight into the atomic rearrangements which take plave when metallic glasses are annealed to low temperature. Such “stabilized” glasses appear to contain structural units which are more regular than those found in rapidly-quenched glasses.
By means of theoretical modeling and experimental synthesis and
characterization, we investigate the structural properties of amorphous
Zr-Si-C. Two chemical compositions are selected, Zr0.31Si0.29C0.40 and
Zr0.60Si0.33C0.07. The amorphous structures are generated in the theoretical
part of our work, by the stochastic quenching (SQ) method, and detailed
comparison is made as regards structure and density of the experimentally
synthesized films. These films are analyzed experimentally using X-ray
absorption spectroscopy, transmission electron microscopy and X-ray
diffraction. Our results demonstrate for the first time a remarkable agreement
between theory and experiment concerning bond distances and atomic coordination
of this complex amorphous metal carbide. The demonstrated power of the SQ
method opens up avenues for theoretical predictions of amorphous materials in
general.
From a theory of Hohenberg and Kohn, approximation methods for treating an inhomogeneous system of interacting electrons are developed. These methods are exact for systems of slowly varying or high density. For the ground state, they lead to self-consistent equations analogous to the Hartree and Hartree-Fock equations, respectively. In these equations the exchange and correlation portions of the chemical potential of a uniform electron gas appear as additional effective potentials. (The exchange portion of our effective potential differs from that due to Slater by a factor of 23.) Electronic systems at finite temperatures and in magnetic fields are also treated by similar methods. An appendix deals with a further correction for systems with short-wavelength density oscillations.
The spin transition in LaCoO3 is investigated by temperature-dependent
resonant soft X-ray emission spectroscopy near the Co 2p absorption edges. This
element-specific technique is more bulk sensitive with respect to the
temperature induced spin-state of the Co3+ ions in LaCoO3 than other
high-energy spectroscopic methods. The spin transition is interpreted and
discussed with ab-initio density-functional theory within the fixed-spin moment
method, which is found to yield consistent spectral functions to the
experimental data. The spectral changes for LaCoO3 as a function of temperature
suggest a change in spin-state as the temperature is raised from 85 to 300 K
while the system remains in the same spin state as the temperature is further
increased to 510 K.
One-, two-, and many-particle calculations for electron-energy-loss near-edge structures (ELNES) are reviewed. The most important point for the ELNES calculation is the proper introduction of the core-hole effect. By introducing the core-hole effect in a sufficiently large supercell, one-particle calculations are applicable to the ELNES of many edges. On the other hand, the two-particle interaction between the excited electron and the core-hole, namely the excitonic effect, is significant in the K edges of very light elements and the L(2,3) edges of Mg and Al. Many-particle interactions, including both electron-electron and electron-hole interactions, are indispensable for the L(2,3) edges of transition metals and the M(4,5) edges of lanthanides, namely white lines. In this review, we present the basics, methodologies, and some applications of one-, two-, and many-particle calculations. In addition, importance of momentum transfer vector in the ELNES calculations for comparison with the experiments is discussed.
We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye-Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.
The electronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has been investigated bysoft x-ray absorption and emission spectroscopy. The measured spectra at the Ti 2p and C 1s thresholds of thenanocomposites are compared to those of Ti metal and amorphous C. The corresponding intensities of theelectronic states for the valence and conduction bands in the nanocomposites are shown to strongly depend onthe TiC carbide grain size. An increased charge transfer between the Ti 3d-eg states and the C 2p states hasbeen identified as the grain size decreases, causing an increased ionicity of the TiC nanocrystallites. It issuggested that the charge transfer occurs at the interface between the nanocrystalline-TiC and the amorphous-Cmatrix and represents an interface bonding which may be essential for the understanding of the properties ofnc-TiC/amorphous C and similar nanocomposites.
Electronic-structure and total-energy calculations are used to elucidate
the properties of the recently synthesized NaCl-structure CrC phase. The
lattice parameter, elastic constants, Γ-point optic-phonon
frequency, and formation energy are determined using total-energy
methods. No sign of lattice instabilities is found, indicating that
NaCl-structure CrC is a true metastable phase. In fact, the elastic
constants and optic-phonon frequency have values comparable to those in
other known NaCl-structure transition-metal carbides. On the other hand,
and in contrast to VC and NbC, CrC is found to be unstable with respect
to phase separation into C and Cr, explaining the difficulty of its
synthesis. The electronic structure is qualitatively similar to that of
TiC and VC except for the placement of the Fermi level EF. A
relatively high density of states at EF, which is derived
from weakly hybridized Cr d states, is found. However, fixed-spin-moment
calculations show that this does not lead to a ferromagnetic
instability. Rigid muffin-tin approximation electron-phonon-interaction
calculations lead to a prediction that CrC is a superconductor with a
transition temperature in the range 5-10 K.
We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.