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.     

Mechanical instability and ideal shear strength of transition metal carbides and nitrides

The ideal shear strength of transition metal carbides and nitrides is calculated with the use of the ab initio pseudopotential density functional method. The microscopic mechanism that limits the ideal strength is studied using full atomic and structural relaxation and the results of electronic structure calculations. It is shown that plasticity in perfect crystals can be triggered by electronic instabilities at finite strains. Our study explicitly demonstrates that the ideal strength in these materials is limited by the elastic instability which is in turn initiated by electronic instabilities. The potential application of alloy hardening due to the onset of instabilities at different strains is also discussed.     

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     

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.     

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.     

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.     

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.     

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.     

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.     

Multiatom interactions,order and stability in binary transition metal alloys

Interface delocalization or depinning transitions such as wetting or surface induced disorder are considered. At these transitions, the correlation length for transverse correlations parallel to the surface diverges. These correlations are studied in the framework of Landau theory. It is shown the t^–1/2 at all types of transitions for systems with short-range forces wheret measures the distance from bulk coexistence.     

Effective electron-electron interactions and magnetic phase transition in a two-dimensional electron liquid

We investigate the spin-dependent effective electron-electron interactions in a uniform system of two-dimensional electrons to understand the spontaneous magnetization expected to occur at very low density. For this purpose, we adopt the Kukkonen-Overhauser form for the effective interactions which are built by accurately determined local-field factors describing the charge and spin fluctuations. The critical behavior of the effective interaction for parallel spin electrons allows us to quantitatively locate the transition to the ferromagnetic state at r_s≈27. When the finite width effects are approximately taken into account the transition occurs at r_s≈30 in agreement with recent quantum Monte Carlo calculations.