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Electronic structure investigation of the cubic inverse perovskite Sc3AlN
Abstract
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.
... At nonresonant 625 eV excitation energy, the L 3 /L 2 branching ratio increases from 1.9 for Cr 0.51 N 0.49 to 4.4 for Cr 0.44 Mo 0.09 V 0.08 N 0.39 . Quantitatively, the significantly higher L 3 /L 2 ratio for the Cr 0.44 Mo 0.09 V 0.08 N 0.39 sample compared with the statistical ratio (2:1) is due to the more effective Coster-Kronig process in conducting systems compared with more localized electrons with less conduction of the Cr 0.51 N 0.49 sample [74]. ...
Chromium-based nitrides are used in hard, resilient coatings and show promise for thermoelectric applications due to their combination of structural, thermal, and electronic properties. Here, we investigate the electronic structures and chemical bonding correlated to the thermoelectric properties of epitaxially grown chromium-based multicomponent nitride Cr(Mo, V)N x thin films. The small amount of N vacancies causes Cr 3d and N 2p states to appear at the Fermi level and reduces the band gap in Cr 0.51 N 0.49. Incorporating holes by alloying of V in N-deficient CrN results in an enhanced thermoelectric power factor with marginal change in the charge transfer of Cr to N compared with Cr 0.51 N 0.49. Further alloying of Mo, isoelectronic to Cr, increases the density of states at the Fermi level due to hybridization of the (Cr, V) 3d and Mo 4d-N 2p states in Cr(Mo, V)N x. This hybridization and N off-stoichiometry result in more metal-like electrical resistivity and reduction in Seebeck coefficient. The N deficiency in Cr(Mo, V)N x also depicts a critical role in reduction of the charge transfer from metal to N site compared with Cr 0.51 N 0.49 and Cr 0.50 V 0.03 N 0.47. In this paper, we envisage ways for enhancing thermoelectric properties through electronic band engineering by alloying and competing effects of N vacancies.
... sample. 68 The N 1s RIXS spectra representing the partial-DOS of N valence region follows the 2p → 1s dipole transitions. The N 1s RIXS spectra excited of all the samples at resonant photon energy of 397.3 eV exhibits a main peak centered around 389.9 eV composed of primarily N 2p states, in agreement with band structure calculations. ...
Chromium-based nitrides are used in hard, resilient coatings, and show promise for thermoelectric applications due to their combination of structural, thermal, and electronic properties. Here, we investigated the electronic structures and chemical bonding correlated to the thermoelectric properties of epitaxially grown chromium-based multicomponent nitride Cr(Mo,V)Nx thin films. Due to minuscule N vacancies, finite population of Cr 3d and N 2p states appear at the Fermi level and diminishes the band opening for Cr0.51N0.49. Incorporating holes by alloying V in N deficient CrN matrix results in enhanced thermoelectric power factor with marginal change in the charge transfer of Cr to N compared to Cr0.51N0.49. Further alloying Mo isoelectronic to Cr increases the density of states across the Fermi level due to hybridization of the (Cr, V) 3d and Mo 4d-N 2p states in Cr(Mo,V)Nx. The hybridization effect with reduced N 2p states off from stoichiometry drives the system towards metal like electrical resistivity and reduction in Seebeck coefficient compensating the overall power factor still comparable to Cr0.51N0.49. The N deficiency also depicts a critical role in reduction of the charge transfer from metal to N site. The present work envisages ways for enhancing thermoelectric properties through electronic band engineering by alloying and competing effects of N vacancies.
... As observed in Fig. 3, the valence band edge shifts to higher energy with increasing x. The hybridization at the top of the valence band shows similarities with XES spectra of other nitrides e.g., Ti2AlN [35] and Sc3AlN [36]. ...
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.
... As observed in Fig. 3, the valence band edge shifts to higher energy with increasing x. The hybridization at the top of the valence band shows similarities with XES spectra of other nitrides e.g., Ti2AlN [35] and Sc3AlN [36]. ...
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.
... Later, Luo et al. [143] investigated the hypothetical Fe-containing 211 MAX-phases. However, more accurate calculations including all competing phases such as inverse perovskites [144] and quaternary phases showed that the Fe-containing 211 system was thermodynamically unstable [145]. Experimentally, a magnetic signal was later observed in Cr 2 − x Mn x GaC phases [146], and attributed to the presence of magnetic perovskite impurities. ...
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.
... Later, Luo et al. [143] investigated the hypothetical Fe-containing 211 MAX-phases. However, more accurate calculations including all competing phases such as inverse perovskites [144] and quaternary phases showed that the Fe-containing 211 system was thermodynamically unstable [145]. Experimentally, a magnetic signal was later observed in Cr 2 − x Mn x GaC phases [146], and attributed to the presence of magnetic perovskite impurities. ...
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 XAS and XES measurements were performed at room temperature (300 K) at the undulator beamline I511-3 at MAX II (MAX-IV Laboratory, Lund, Sweden), comprising a 49-pole undulator, and a modified SX-700 plane grating monochromator [21,22]. The measurements were made at a base pressure lower than 6.7*10 −7 Pa. ...
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.
... [10][11][12] In particular, in Ti 2 AlN the states that are close to the Fermi level (À1.1 eV) are mainly due to Ti 3d-Al 3p bonding, while in the case of Sc 3 AlN the bandgap is closed with the Sc 3d states. 13,14 Finally, a great amount of work has been performed on nitride-based interfaces and heterostructures such as AlN/ GaN and Al 1-x In x N/GaN that have a larger refractive index and bandgap in comparison to and may improve the efficiency of light emitting diodes, high electron mobility transistors, and heterojunction field effect transistors. [18][19][20] Recently, a metallo-dielectric nanocomposite AlN coating containing transition-metal nanocluster inclusions (Ag) has been successfully grown by pulsed laser deposition. ...
We present theoretical and experimental results on the bonding and structural characteristics of AlN:Ag thin film nanocomposites obtained by means of density functional theory (DFT) computations, high resolution transmission electron microscopy (HRTEM) observations, Auger electron spectroscopy (AES), and x-ray diffraction (XRD) measurements. From the theoretical calculations it was determined that the presence of the Ag substitutional of N or Al atoms affects the electronic density of states (EDOS) of the resulting systems. In particular, occupied energy states are introduced (between others) that lie within the energy gap of the AlN matrix due to Ag-d, Al-p (accompanied with a charge transfer from Al to Ag), Ag-p, and N-p hybridizations, respectively. The effect is predicted to be even more pronounced in the case of Ag nanoparticle inclusions affecting the EDOS of the composite system. These predictions were verified by the HRTEM images that gave unequivocal evidence for the presence and stability of Ag nanoparticles in the AlN matrix. In addition, the AES data suggested a metal-metal (Ag-Al) bonding preference, while the XRD patterns revealed that the atomic Ag dispersions in the AlN thin films results in a small elongation of the Wurtzite lattice, which is in agreement with the DFT predictions. These results may useful in tailoring the electronic response of AlN-based systems and the design of devices for various opto-electronic applications.
In recent decades, MAX phases have attracted considerable attention from the scientific community due to their unique combination of metallic and ceramic properties, which provide exceptional mechanical, thermal, electrical and chemical characteristics. The synthesis of MAX phases in the form of coatings is of increasing interest for many applications. The aim of this review is to summarize the progress made in the synthesis of coatings based on MAX phases using different methods. The advantages and characteristics of the implementation of ion-plasma physical vapor deposition methods are discussed. The use of ion-plasma methods allows to significantly reduce the synthesis temperature of MAX phases due to the high energy of the particles forming the coating. The effect of deposition parameters on the composition, structure and properties of the coatings is analyzed. Coatings with high protective properties and prospects for their application in industry are considered. This part of the review focuses on methods for depositing MAX phase based coatings.
In the last two decades, there has been a renewed interest in the chemistry of nitrides and nitridometalates. Both binary and higher nitrides M x N y have already featured prominently as refractory materials, corrosion‐ and mechanical wear‐resistant coatings, hard materials, and hard magnets; thin films are used as diffusion barriers in integrated circuits.
Whereas research on binary transition metal nitrides is mostly driven by technical and economic interests, the investigation on nitridometalates primarily focuses on exploration with respect to the development of new synthetic strategies and the design of new materials. Within this field of interest, especially the nitride chemistry of rare earth metals is still comparably undeveloped.
Chemical bonding in binary nitrides varies from primarily salt‐like via covalent to metallic, whereas nitridometalates are best described as containing covalent complex anions [ M x N y ] z ⁻ with alkali, alkaline earth, or rare earth metal cations providing electroneutrality.
The investigations of structural, elastic, electronic and magnetic properties of ferromagnetic antiperovskite Pr3InO are carried out using the full-potential linearised augmented plane waves (FP-LAPW) method, in the framework of the density functional theory (DFT), within the Wien2k code. The exchange–correlation potential was treated by means of LSDA, GGA-PBE and GGA-PBEsol approximations. The calculated band structure and density of states with the GGA-PBE approximation reveal that Pr3InO alloy is metallic. The total magnetic moment is non-integer confirming the ferromagnetic metallic behaviour of Pr3InO alloy. Elastic parameters, performed within GGA-PBE and LSDA approximations, show that Pr3InO alloy is stable in the cubic structure with brittle nature. It is found that the investigated compound exhibits an anisotropic behaviour. There are no available theoretical or experimental studies devoted to this material. Thus, our results are pure prediction which can serve as reference for furthers works.
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.
Semi-conductive MXene Sc2COx is synthesized by using magnetron sputtering. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) is performed to determine the composition of sample. The bandgap of Sc2COx sample is determined by ultraviolet-visible (UV-VIS) range absorption and photoluminescence (PL) spectrum. The Tauc plot indicates that Sc2COx is a direct bandgap material. The high sensitivity of scandium to oxygen makes the sample contain both scandium carbide and scandium oxide. Thermal annealing introduces more oxidation into the sample. Structure of MXene will be broken when the annealing temperature is higher than 600°C.
The structural properties, surface energies and electronic structure of SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO inverse-perovskite are investigated using first-principles calculations. Comparison of the atomic relaxations of SrSn and Sr2O layers of the (001) surfaces of Sr3SnO shows that largest atomic relaxation is achieved in the 1st layer Sn atom of the SrSn-terminated surface which dissipates as one moves toward the center of the slab. Moreover, largest surface rumpling and change in the inter-layer distances are found for SrSn-terminated surface. Cleavage, relaxation and surface energies are computed to examine stability of SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO. The electronic properties of the bulk and SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO are studied using electronic band structure and density of states. We find a conducting behavior for both SrSn- and Sr2O-terminated (001) surfaces of Sr3SnO which is mainly caused by the Sn-5p states.
We report the epitaxial growth of Ca3SnO antiperovskite oxide films on (001)-oriented cubic yttria-stabilized zirconia (YSZ) substrates by using a conventional pulsed laser deposition (PLD) technique. In this work, a sintered Ca3SnO pellet is used as the ablation target. X-ray diffraction measurements demonstrate the (001) growth of Ca3SnO films with the antiperovskite structure and a cube-on-cube orientation relationship to the YSZ substrate. The successful synthesis of the antiperovskite phase is further confirmed by x-ray photoemission spectroscopy. These results strongly suggest that antiperovskite-oxide films can be directly grown on substrates from the target material using a PLD technique.
The structural stability, electronic structural and mechanical properties of Ti3AlCx N1−x (0 ≤ x ≤ 1) has been investigated using the first-principles pseudopotential plane-wave method. The results for the formation energy of these compounds indicate that all the structures are stable. The equilibrium lattice constant values of Ti3AlC and Ti3AlN compounds are in good agreement with the experimental and theoretical data. Bonding behaviour of these alloys has been explained based on Cauchy pressure, electronic density of states and charge density. The Ti3AlN and Ti3AlCx N1−x alloys show metallic bonding whereas Ti3AlC and Ti3Al display directional bonding, judged from Cauchy pressure values. All the structures satisfy the Born stability criteria in terms of elastic constants. Based on the G/B ratios, all structures are associated with ductile behaviour except Ti3Al and Ti3AlC which are brittle.
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.
The electronic structure of cubic inverse perovskites Gd3MX (M=Al, Ga and In; X=B, C, N and O) are investigated with the first-principles method. The ground-state structure, equilibrium structural parameters, and magnetic structure are calculated; a comparison between these compounds is made based upon follow-on analysis. The electronic structures reveal that all these compounds have a metallic character. We report a detailed analysis of the different physical properties and their changes induced by varying the cation and/or the anion.
Based on first-principles approach, we present a comparative study of structural, electronic, elastic and thermo-dynamical properties of the series of inverse-perovskites Sc3AC, with A = Al, Ga, In and Tl. The calculated equilibrium lattice constants are in excellent agreement with the experimental and available theoretical data. The electronic band structures and densities of states profiles show that the studied compounds are conductors. Analysis of atomic site projected local density of states and charge densities reveals that a mixture of covalent–ionic–metallic characterizes the chemical bonding of the considered inverse-perovskites. Pressure dependence up to 40 GPa of the single-crystal and polycrystalline elastic constants has been investigated in details. The computed B/G ratios show that all Sc3AC compounds are brittle. We have estimated the sound velocities in the principal directions. Through the quasi-harmonic Debye model, in which the phononic effects are taken into account, the temperature and pressure effects on the lattice constant, bulk modulus, heat capacity and Debye temperature are performed.
X-ray photoelectron XPS, ultraviolet photoelectron UPS, and Auger electron spectroscopy AES spectra are presented from epitaxial, single-crystal transition-metal TM nitride ScN, TiN, VN, and CrN layers, that were grown in situ in an ultrahigh-vacuum UHV magnetron sputter deposition system attached to the analysis chambers. The samples are near-stoichiometric with N/Me ratios determined by Rutherford backscattering spectroscopy RBS, and contain no bulk or surface impurities detectable by XPS, AES, or RBS. The spectra therefore represent reliable reference data. We also present XPS and AES data from sputter-etched samples in order to quantitatively determine the effect of preferential sputtering of nitrogen in TM nitrides. The sputter etching was performed under conditions typical for sputter cleaning of air-exposed samples until a steady state N/Me ratio is reached. © 2000 American Vacuum Society. Transition metal TM nitrides are well known for their remarkable physical properties including high hardness, physical strength, chemical inertness, and high temperature stability. As a result, they have become technologically im-portant for applications such as hard wear-resistant coat-ings, diffusion barriers, and optical coatings. NaCl-structure TM nitrides exhibit large single-phase fields, i.e., they can sustain large vacancy concentrations on both cation and anion sublattices and are stable in the NaCl structure over a large composition range. TiN, for example, is stable for N/Ti ratios ranging from 0.6 to 1.2. Thus, a common chal-lenge when depositing and analyzing these functional TM TABLE I. Peak positions obtained from in situ XPS analyses "Mg K excitation… of as-deposited epitaxial ScN, TiN, VN, and CrN layers. The spectra were fit with Doniach–Sunjic "2p… and Voight "N 1s… functions. NÕmetal ratios before and after Ar ion sputtering, as determined by XPS using sensitivity factors from Ref. 2, are com-pared with bulk data obtained by RBS.
We have previously shown that one-particle calculations of X-ray spectra in simple metals give very different results depending on whether the core hole is taken into account or not. Guided by experiment and the dynamical theory of X-ray spectra developed by Nozières and DeDominicis (ND) we have, however, also established the rule that rather realistic spectra can be obtained from a one-particle calculation provided final state wavefunctions are used in the transition matrix elements. In this paper we report on direct numerical evaluations of the dynamical ND theory for several different cases including the case where a bound state appears. In all cases the results support our final state rule. We also give arguments in support of the procedure used to extract threshold exponents and asymmetry indices from X-ray absorption and emission spectra and X-ray photoemission spectra.
ScN(001) 1×1 surfaces have been prepared by growing ScN on MgO(001) using radio frequency molecular beam epitaxy. In situ ultrahigh vacuum scanning tunneling spectroscopy indicates that the Fermi level at the surface lies slightly above the Sc 3d conduction band edge, which is attributed to a downward band bending at the surface. In situ scanning tunneling microscopy is used to image the Sc and N atom sublattices. While only one atom (Sc) appears at small negative bias, both atoms (Sc and N) appear at certain positive sample biases due to the partially ionic nature of the bonding. Charge accumulation near ionized subsurface donors is evident from the long-range topographic distortions at the surface. The combination of tunneling spectroscopy and optical absorption results show that ScN has an indirect bandgap of 0.9±0.1 eV and a direct transition at 2.15 eV.
Mechanical and thermodynamic stability of the isoelectronic ternary inverse perovskites Sc3EN (E=B,Al,Ga,In) has been studied from first principles. We confirm stability of recently synthesized cubic phases Sc3AlN and Sc3InN, and predict the stability of cubic Sc3GaN and a triclinic phase aP20-Sc3BN. Substantial phonon softening in Sc3AlN and Sc3GaN is observed indicating a possibility that structural defects could form readily. In accord, our experiments show that magnetron sputter deposited films contain regions with high density of nonperiodic stacking faults along the ⟨111⟩ growth direction. We suggest that defect-free crystals may exhibit anomalies in the carrier properties, promising for electronic applications.
Experimental and ab initio computational methods are employed to conclusively show that ScN is a semiconductor rather than a semimetal; i.e., there is a gap between the N 2p and the Sc 3d bands. Previous experimental investigators reported, in agreement with band structure calculations showing a band overlap of 0.2 eV, that ScN is a semimetal while others concluded that it is a semiconductor with a band gap larger than 2 eV. We have grown high quality, single crystalline ScN layers on MgO(001) and on TiN(001) buffer layers on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. ScN optical properties were determined by transmission, reflection, and spectroscopic ellipsometry while in-situ x-ray and ultraviolet valence band photoelectron spectroscopy were used to determine the density of states (DOS) below the Fermi level. The measured DOS exhibits peaks at 3.8 and 5.2 eV stemming from the N 2p bands and at 15.3 eV due to the N 2s bands. The imaginary part of the measured dielectric function ɛ2 consists of two primary features due to direct X- and Γ-point transitions at photon energies of 2.7 and 3.8 eV, respectively. For comparison, the ScN band structure was calculated using an ab initio Kohn–Sham approach which treats the exchange interactions exactly within density-functional theory. Calculated DOS and the complex dielectric function are in good agreement with our ScN valence-band photoelectron spectra and measured optical properties, respectively. We conclude, combining experimental and computational results, that ScN is a semiconductor with an indirect Γ–X bandgap of 1.3±0.3eV and a direct X-point gap of 2.4±0.3eV.
We present the results of Kohn-Sham calculations on molecules, surfaces, and solids which were obtained using a recently proposed exchange-correlation energy functional [ Z. Wu and R. E. Cohen Phys. Rev. B 73 235116 (2006)]. The Wu-Cohen (WC) functional, like the well-known PBE functional [ J. P. Perdew et al. Phys. Rev. Lett. 77 3865 (1996)], is of the generalized gradient approximation form and was derived from the homogeneous electron gas and mathematical relations obeyed by the exact functional. The results on molecular systems show that among the functionals we tested, PBE remains superior for the energetics of covalent and noncovalent bonds. While this is not too surprising for noncovalent bonds due to the very good performance of PBE, unfortunately this holds also for covalent bonds, where PBE is a functional of rather poor quality. Calculations on transition-metal surfaces show that WC improves over local-density approximation (LDA) and PBE for the surface formation energy of 3d elements, while LDA is the best for heavier elements. In most cases, the lattice constant of solids as determined by the WC functional is in between the LDA and PBE results and on average closer to experiment. We show for each group of compounds which functional performs best and provide trends. In the particular case of lattice constants whose values are determined by weak interactions (e.g., the interlayer distance in graphite), the LDA functional is more accurate than the generalized gradient approximation functionals.
We have produced pure thin-film single-crystal Ti2AlN(0001), a member of the Mn+1AXn class of materials. The method used was UHV dc reactive magnetron sputtering from a 2Ti:Al compound target in a mixed Ar–N2 discharge onto (111) oriented MgO substrates. X-ray diffraction and transmission electron microscopy were used to establish the hexagonal crystal structure with c and a lattice parameters of 13.6 and 3.07 Å, respectively. The hardness H, and elastic modulus E, as determined by nanoindentation measurements, were found to be 16.1±1 GPa and 270±20 GPa, respectively. A room-temperature resistivity for the films of 39 μΩ cm was obtained.
Several improvements of the tetrahedron method for Brillouin-zone integrations are presented. (1) A translational grid of k points and tetrahedra is suggested that renders the results for insulators identical to those obtained with special-point methods with the same number of k points. (2) A simple correction formula goes beyond the linear approximation of matrix elements within the tetrahedra and also improves the results for metals significantly. For a required accuracy this reduces the number of k points by orders of magnitude. (3) Irreducible k points and tetrahedra are selected by a fully automated procedure, requiring as input only the space-group operations. (4) The integration is formulated as a weighted sum over irreducible k points with integration weights calculated using the tetrahedron method once for a given band structure. This allows an efficient use of the tetrahedron method also in plane-wave-based electronic-structure methods.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
The optical conductivity is evaluated for interband transitions between a flat valence band and a parabolic conduction band. The conduction band is filled with electrons to a Fermi energy muF. The conductivity is calculated assuming that the electron-hole interaction is attractive, static, and short range. The final-state interactions between the electron and hole cause a divergence in the conductivity at the interband threshold. This divergence appears to go as a power law. For this case of an infinite hole mass, the exciton binding energy vanishes, since the singularity in the scattering amplitude occurs just at threshold.
Titanium was reactively r.f. sputtered in mixed ArN2 and ArCH4 discharges onto substrates held at 775 K. The films obtained were characterized by scanning electron microscopy and X-ray diffraction and through measurements of the microhardness and electrical resistivity. The composition of the films was determined by Auger electron spectroscopy. The measurements show that the morphology of the deposits to a large extent influences the properties of the films obtained. For TiN coatings the electrical resistivity reaches the bulk resistivity only if coatings with the full bulk density are obtained. The difference observed in the lattice parameter for TiN thin film and bulk samples is explained using a grain boundary relaxation model. It is also shown that the heat of formation of the compounds plays an important role in the formation of carbide and nitride films. A high heat of formation promotes the development of large grains and dense structures.
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.
The singularities of x-ray absorption or emission in metals are studied by a new "one-body" method, which describes the scattering of conduction electrons by the transient potential due to the deep hole. Using the linked-cluster theorem, the net transition rate in the time representation is expressed as the product of two factors: a one-electron transient Green's function L, and the deep-level Green's function G. These factors obey simple Dyson equations, which can be solved asymptotically by using Muskhelishvili's method. The x-ray transition rate is found to behave as 1εalpha, where ε is the frequency measured from the threshold, and alpha an exponent involving the various phase shifts deltal which describe scattering by the deep hole. alpha may be >0 (infinite threshold) or
We present a nonempirical density functional generalized gradient approximation (GGA) that gives significant improvements for lattice constants, crystal structures, and metal surface energies over the most popular Perdew-Burke-Ernzerhof (PBE) GGA. The functional is based on a diffuse radial cutoff for the exchange hole in real space, and the analytic gradient expansion of the exchange energy for small gradients. There are no adjustable parameters, the constraining conditions of PBE are maintained, and the functional is easily implemented in existing codes.
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 of the heavy fermion compound CeB6 is probed by resonant inelastic soft-x-ray scattering using photon energies across the Ce 3d and 4d absorption edges. The hybridization between the localized 4f orbitals and the delocalized valence-band states is studied by identifying the different spectral contributions from inelastic Raman scattering and normal fluorescence. Pronounced energy-loss structures are observed below the elastic peak at both the 3d and 4d thresholds. The origin and character of the inelastic scattering structures are discussed in terms of charge-transfer excitations in connection to the dipole allowed transitions with 4f character. Calculations within the single-impurity Anderson model with full multiplet effects are found to yield consistent spectral functions to the experimental data.
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.
The compounds (R3N)In (R=Sc, La–Nd, Sm, Gd–Tm, Lu) were synthesized by arc melting and sintering reactions from the binaries RN and R2In. Combined Rietveld refinements on X-ray and neutron powder diffraction data on the Ce-phase and X-ray diffraction for the other phases reveal the compounds (R3N)In to crystallize in the perovskite structure ((Ce3N)In: , a=504.89(2) pm; Rietveld: neutron powder diffraction: RF=0.056, RBragg=0.104, X-ray powder diffraction: RF=0.069, RBragg=0.115; refined composition Ce3InN0.92(1), chemical analysis Ce3InN0.91(2)O0.05(1)). Compounds R3In (AuCu3 structure type) are known for R=La–Sm. Nitrogen is able to stabilize this AuCu3-substructure with R=Gd–Tm, Lu and Sc. In measurements of the magnetic susceptibility (Ce, Pr, Nd, Sm, Tb: antiferromagnetic ordering at low temperature) and X-ray absorption spectroscopy at the R-LIII-edges the rare-earth atoms behave like R3+ species in insulating compounds. The electrical resistivity of the Sc, Ce and Nd compounds shows metallic temperature dependence. Chemical bonding was analyzed via LDA-COHP calculations for (La3N)In showing ionic polyhedra NLa6/2 embedded in a metallic matrix.
For narrow band metals the distortions of the form of X-ray and X-ray photoelectron spectra are investigated under the influence of the vacancy on the inner level. A calculation method over the whole energy interval of the distoring influence of the hole field is proposed and realized numerically. The site formalism is developed to describe the X-ray spectra. Equations are obtained in the energetic representation which allow to effect a numerical analysis.
Für Schmalbandmetalle werden die Verzerrungen der Form der Röntgen- und Röntgenphoto-elektronenspektren unter dem Einfluß einer Lücke auf das innere Niveau untersucht. Es wird eine Berechnungsmethode über das gesarute Energieintervall des Verzerrungseinflusses des ge-samten Feldes vorgeschlagen und numerisch realisiert. Der Platzformalismus zur Beschreibung der Röntgenspektren wird beschrieben. Gleichungen in der Energiedarstellung werden erhalten, die eine nuraerische Analyse ermöglichen.
Sc3AlN with perovskite structure has been synthesized as the first ternary phase in the Sc–Al–N system. Magnetron sputter epitaxy at 650 °C was used to grow single-crystal, stoichiometric Sc3AlN(111) thin films onto MgO(111) substrates with ScN(111) seed layers as shown by elastic recoil detection analysis, X-ray diffraction, and transmission electron microscopy. The Sc3AlN phase has a lattice parameter of 4.40 Å, which is in good agreement with the theoretically predicted 4.42 Å. Comparisons of total formation energies show that Sc3AlN is thermodynamically stable with respect to all known binary compounds. Sc3AlN(111) films of 1.75 µm thickness exhibit a nanoindentation hardness of 14.2 GPa, an elastic modulus of 249 GPa, and a room-temperature electrical resistivity of 41.2 µΩ cm. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
ScN crystals were grown on tungsten foil by sublimation-recondensation method in the temperature range of 1840-2060degreesC, pressure range of 15-230 Torr under a nitrogen atmosphere. The growth rate increased exponentially with temperature with activation energy of 456.0 KJ/mol, and it was inversely proportional to pressure. The maximum growth rate was 79.292 mg/h at 2060degreesC under 25 Torr. Characterization methods confirmed the rock salt crystal structure with a lattice constant of 4.5005 Angstrom. (C) 2004 Kluwer Academic Publishers.
A formalism to compute x-ray spectra due to core excitations in metals by using single-particle band-structure techniques is presented and illustrated with a detailed calculation of the $K,L$, and $M$ emission and absorption spectra of palladium over 200 eV. Within the muffin-tin approximation for the potential, any spectrum can be factorized into atomiclike and solid-state contributions. The atomiclike factor is the dipole transition strength connecting a core state to a muffin-tin orbital in a free-electron metal. The solid-state factor is proportional to the density of band states with angular momentum determined by the orbital symmetry of the core state and the dipole selection rules. These projected densities of states have been calculated by using a linearized version of the augmented-plane-wave method specifically designed to cover large energy ranges. In particular, the method can describe simultaneously several principal quantum numbers of the eigenstates (e.g., $4d$ and $5d$ for palladium).
The mechanical properties of (001)‐, (011)‐, and (111)‐oriented MgO wafers and 1‐μm‐thick TiN overlayers, grown simultaneously by dc magnetron sputter deposition at 700 °C in a mixed N 2 and Ar discharge, were investigated using nanoindentation. A combination of x‐ray‐diffraction (XRD) pole figures, high‐resolution XRD analyses, and Auger electron spectroscopy was used to show that all TiN films were single crystals with N/Ti ratios of 1.0±0.05. The nanoindentation measurements were carried out using a three‐sided pyramidal Berkovich diamond indentor tip operated at loads ranging from 0.4 to 40 mN. All three orientations of MgO substrates, as‐received, exhibited identical hardness values as determined using the Oliver and Pharr method. After a 1 h anneal at 800 °C, corresponding to the thermal treatment received prior to film growth, the measured hardness of MgO(001) was 9.0±0.3 GPa. All TiN films displayed a completely elastic response at low loads. Measured hardness values, which decreased with increasing loads, increased in the order (011)≪(001)≪(111). After a 30 s postdeposition anneal at 1000 °C, however, hardness was found to be independent of load except at displacements ≫100 nm where substrate effects were apparent. TiN(001) and (111) films had hardnesses of 20±0.8 and 21±1 GPa, respectively, while data obtained from (011) layers exhibited large scatter due to surface roughness effects. Young’s moduli for annealed samples, calculated from the elastic unloading curves, were found to be 307±15 GPa for MgO (001) and 445±38 and 449±28 GPa for TiN (001) and TiN (111), respectively. © 1996 American Institute of Physics.
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the-
different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
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.
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 phase equilibria in the ternary system titanium-aluminum-nitrogen are investigated for two isothermal sections. At 1273 K one encounters the H-phase Ti2AlN (a = 0.29912 nm, c = 1.3621 nm) and the cubic perovskite-type phase Ti3AlN (a = 0.41120 nm). At 1573 K one encounters additionally Ti3Al2N2 (space group: P31c, hexagonal axes a = 0.29875 nm, c = 2.3350 nm). α-Titanium dissolves nitrogen and aluminum to a large extent, but no solubility of the third element was detected in any of the other binary phases.
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.
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 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 electronic structure of the heavy fermion compound CeB6 is probed by
resonant inelastic soft X-ray scattering using photon energies across the Ce 3d
and 4d absorption edges. The hybridization between the localized 4f orbitals
and the delocalized valence-band states is studied by identifying the different
spectral contributions from inelastic Raman scattering and normal fluorescence.
Pronounced energy-loss structures are observed below the elastic peak at both
the 3d and 4d thresholds. The origin and character of the inelastic scattering
structures are discussed in terms of charge-transfer excitations in connection
to the dipole allowed transitions with 4f character. Calculations within the
single impurity Anderson model with full multiplet effects are found to yield
consistent spectral functions to the experimental data.
Resonant soft X-ray Raman scattering measurements on NiO have been made at
photon energies across the Ni 2p absorption edges. The details of the spectral
features are identified as Raman scattering due to d-d and charge-transfer
excitations. The spectra are interpreted within the single impurity Anderson
model, including multiplets, crystal-field and charge-transfer effects. At
threshold excitation, the spectral features consists of triplet-triplet and
triplet-singlet transitions of the 3d8 configuration. For excitation energies
corresponding to the charge-transfer region in the Ni 2p X-ray absorption
spectrum of NiO, the emission spectra are instead dominated by charge-transfer
transitions to the 3d9L-1 final state. Comparisons of the final states with
other spectroscopical techniques are also made.
We report resonant inelastic x-ray scattering (RIXS) excited by circularly
polarized x-rays on Mn-Zn ferrite at the Mn L2,3-resonances. We demonstrate
that crystal field excitations, as expected for localized systems, dominate the
RIXS spectra and thus their dichroic asymmetry cannot be interpreted in terms
of spin-resolved partial density of states, which has been the standard
approach for RIXS dichroism. We observe large dichroic RIXS at the L2-resonance
which we attribute to the absence of metallic core hole screening in the
insulating Mn-ferrite. On the other hand, reduced L3-RIXS dichroism is
interpreted as an effect of longer scattering time that enables spin-lattice
core hole relaxation via magnons and phonons occurring on a femtosecond time
scale.
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\|.
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