Effective pair interactions in transition metal carbides and nitrides

Effective pair interactions up to the 4-th nearest neighbors have been calculated for TiC_x, NbC_x, TiN_x and NbN_x as a function of the vacancy concentration using the KKR-CPA implementation of the generalized perturbation theory. While for the carbides good agreement with experimental data was found, for the nitrides the theory fails to predict the type of ordering presently accepted in the literature.     

Vacancy hardening and softening in transition metal carbides and nitrides

The effects of vacancies on mechanical properties of the transition metal carbides and nitrides are studied using the ab initio pseudopotential approach. Calculated shear elastic stiffness and electronic structures show that the vacancy produces entirely different effects on the mechanical strength of groups IVb nitrides and Vb carbides. It is found that the occupation of shear-unstable metallic dd bonding states changes essentially in an opposite way for the carbides and nitrides in the presence of vacancies, resulting in different responses to shear stress. Our study provides an atomistic understanding of the anomaly in hardness for these substoichiometric materials.     

Vacancy-induced changes in the electronic structure of transition metal carbides and nitrides: Calculation of X-ray photoemission intensities

Starting from KKR-CPA and KKR-GF densities of states and cross-sections within the single-scatterer final-state approximation, X-ray photoemission intensities were calculated for a series of stoichiometric and substoichiometric transition metal carbides and nitrides. For all compounds nonmetal vacancies give rise to an additional peak in the XPS spectra. The theoretical results are compared to several experimental XPS measurements. In most cases very good agreement is found. The discrepancies between theory and experiment are discussed in detail.Dedicated to Prof. A. Neckel on the occasion of his 60th birthday     

Neutron diffraction studies of transition metal nitrides

The magnetic structures of Fe₄N and Mn₄N have been redetermined using neutron diffraction. The magnetic form factors, obtained from polarised neutron data have been shown to be different for the face-centred and corner atoms. A qualitative explanation of the structures of Fe₄N and Mn₄N has been provided from the shapes of the magnetic form factors. Since deceased.     

Acoustical and elastic properties of transition metal nitrides

Elastic moduli of transition metal nitrides, TiN, ZrN and HfN, have been evaluated using electrostatic and Born repulsive potentials. Acoustical dissipation due to phonon-phonon (p-p) interaction, thermoelastic mechanism and dislocation damping due to screw and edge dislocations has been evaluated in the temperature range 50-500 K along the three crystallographic directions of propagation, viz. [1 0 0], [1 1 0] and [1 1 1] for longitudinal and shear modes. Grüneisen numbers, acoustic coupling constants and their ratios have been evaluated for the longitudinal and shear waves. Results are in good agreement with available data.     

Antiferromagnetic half-metallicity of transition metal nitrides under volume expansion

The transition metal pnictides ABX₂ have been recently proposed as half-metallic fully compensated ferrimagnets, also briefly referred to as half-metallic “antiferromagnets” [N. Long, M.Ogura, H. Akai, J. Phys.: Condens. Matter 21, 064241 (2009)]. In this work we carry out a systematic study of the more promising cases of the transition metal nitrides MnCoN₂ and NiCrN₂ on the basis of density functional theory in the framework of full-potential linearized augmented plane wave method. The electronic structures and the magnetic properties of the above hypothetical compounds in Zinc-blende-type, NaCl-type, and Wurtzite-type structure are calculated within generalized gradient approximation. The results reveal that, although these compounds are metallic in their bulk equilibrium in all three structures, they exhibit antiferromagnetic half-metallicity under negative stress or volume expansion in a limited range of lattice parameters, which is significantly larger than the equilibrium values. This suggests that a situation in which half-metallicity may arise, is when these compounds are coated on semiconducting layers of larger lattice constant.     

Electronically driven phonon anomalies in transition metal carbides

Large maxima in the generalized susceptibility of χ(q) of TaC calculated from its APW band structure are found to correlate very well with observed soft phonon modes. Such maxima are not found for HfC where phonon anomalies are not observed.     

Phonon anomalies in transition metal nitrides: δ-NbN

The phonon dispersion curves in δ-NbN have been measured at room temperature in the high symmetry directions Δ, Σ, and ? by coherent inelastic neutron scattering. Anomalies in the dispersion of the acoustic branches have been detected which are quite different from those which have already been reported for superconducting transition metal compounds with 9 valence electrons. In the 10 valence electron compound δ-NbN the soft mode region has moved from the middle of the Σ-direction to the zone boundary. The experimental results are well described by a double shell model which has also been used to calculate the density of states. Using the formalism of Varma and Weber the measured dispersion curves have been reproduced. This shows that the change in the anomalies when going from 9 to 10 valence electrons, is due to a shift in Fermi energy in an essentially rigid band scheme.     

High pressure behavior and structural properties of transition metal carbides

A pressure induced structural phase transition from NaCl-type (B₁) to CsCl-type (B₂) structure has been predicted in transition metal carbides, namely TiC, ZrC, NbC, HfC, and TaC by using an interionic potential theory with modified ionic charge (Z_m ), which includes Coulomb screening effect due to d-electron. The phase transition pressure (P_T ) relies on large volume discontinuity in pressure–volume relationship, and identifies the structural phase transition from B₁ phase to B₂ phase. The variation of second-order elastic constants with pressure follows a systematic trend identical to that observed in other compounds of NaCl-type structure. The Born criterion for stability is found to be valid in transition metal carbides.     

Synthesis of novel transition metal nitrides IrN2 and OsN2

Two new transition metal nitrides, IrN2 and OsN2, were synthesized at high pressures and temperatures using laser-heated diamond-anvil cell techniques. Synchrotron x-ray diffraction was used to determine the structures of novel nitrides and the equations of states of both the parent metals as well as the newly synthesized materials. The compounds have bulk moduli comparable with those of the traditional superhard materials. For IrN2, the measured bulk modulus [K0 = 428(12) GPa] is second only to that of diamond (K0 = 440 GPa). Ab initio calculations indicate that both compounds have a metal:nitrogen stoichiometry of 1:2 and that nitrogen intercalates in the lattice of the parent metal in the form of single-bonded N-N units.     

Mechanical properties and electronic structure of the incompressible rhenium carbides and nitrides: A first-principles study

By means of first-principles calculations, the structural stability, mechanical properties and electronic structure of the newly synthesized incompressible Re₂C, Re₂N, Re₃N and an analogous compound Re₃C have been investigated. Our results agree well with the available experimental and theoretical data. The proposed Re₃C is shown to be energetically, mechanically and dynamically stable and also incompressible. Furthermore, it is suggested that the incompressibility of these compounds is originated from the strong covalent bonding character with the hybridization of 5d orbital of Re and the 2p orbital of C or N, and a zigzag topology of interconnected bonds, e.g., Re-Re, Re-C or Re-N bonding.     

d-band effects in the lattice dynamics of transition metal carbides

We present the first ab initio. calculation of the d-bandstructure contribution to the acoustic phonon frequencies of NbC. The results, obtained. by freezing in the lattice vibrations within the rigid muffin tin approximation, give anomalies in the longitudinal modes in agreement with experiment. The position of the 〈001〉 and 〈111〉 soft modes are sensitive to the band filling. The transverse anomalies are not explained by pure d-interactions, highlighting the importance of p-d hybridization in the carbides.     

Microscopic analysis of the characteristic energy loss spectra of 3d-and 4d-transition metal carbides and nitrides

Microscopic studies of the electron-energy-loss-spectra (EELS) of the nitrides and carbides of the transition metals Ti, V, Zr, Nb in the energy range to about 35 eV have been done using the linear-muffin-tin-orbital (LMTO) method for solving the band structure problem. Special attention has been paid to the low energy region where for some of these compounds there is an interesting feature in the EELS. Good agreement between the calculations and experiment was obtained. The similarities and differences of these compounds are discussed in terms of their electronic structure.     

First principles study on the charge density and the bulk modulus of the transition metals and their carbides and nitrides

A first principles study of the electronic properties and bulk modulus （B0） of the fcc and bcc transition metals, transition metal carbides and nitrides is presented. The calculations were performed by plane-wave pseudopotential method in the framework of the density functional theory with local density approximation. The density of states and the valence charge densities of these solids are plotted. The results show that B0 does not vary monotonically when the number of the valence d electrons increases. B0 reaches a maximum and then decreases for each of the four sorts of solids. It is related to the occupation of the bonding and anti-bonding states in the solid. The value of the valence charge density at the midpoint between the two nearest metal atoms tends to be proportional to B0.     

Mechanical properties of lateral transition metal dichalcogenide heterostructures

Transition metal dichalcogenide(TMD)monolayers attract great attention due to their specific structural,electronic and mechanical properties.The formation of their lateral heterostructures allows a new degree of flexibility in engineering electronic and optoelectronic dervices.However,the mechanical properties of the lateral heterostructures are rarely investigated.In this study,a comparative investigation on the mechanical characteristics of 1H,IT'and 1H/1T'heterostructure phases of different TMD monolayers including molybdenum disulfide(M0S₂)molybdenum diselenide(MoSe₂),Tungsten disulfide(WS₂),and Tungsten diselenide(WSe₂)was conducted by means of density functional theory(DFT)calculations.Our results indicate that the impact of the lateral heterostructures has a relatively weak mechanical strength for all the TMD monolayers.The significant correlation bet ween the mechanical properties of the TMD monolayers and their structural phases can be used to tune their stiffness of the materials.Our findings,therefore,suggest a novel strategy to manipulate the mechanical characteristics of TMDs by engineering their structural phases for their practical applications.     

Controlling spin-orbit coupling strength of bulk transition metal dichalcogenide semiconductors

Transition metal dichalcogenide (TMD) semiconductors are attracting much attention in research regarding device physics based on their unique properties that can be utilized in spintronics and valleytronics. Although current studies concentrate on the monolayer form due to the explicitly broken inversion symmetry and the direct band gap, bulk materials also hold the capability of carrying spin and valley current. In this study, we report the methodology to continuously control the spin-orbit coupling (SOC) strength of bulk TMDs Mo_1-xW_xSe₂ by changing the atomic ratio between Mo and W. The results show the size of band splitting at the K valley the measure of the coupling strength is linearly proportional to the atomic ratio of Mo and W. Our results thus demonstrate how to precisely tune the SOC coupling strength, and the collected information of which can serve as a reference for future applications of bulk TMDs.     

Thermodynamic ground states of platinum metal nitrides

The thermodynamic stabilities of various phases of the nitrides of the platinum-metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir, and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed.