Xiao-Fen Huang

Pressure-induced structural transition and thermodynamic properties of NbN and effect of metallic bonding on its hardness

Using first- principles calculations, the elastic constants, thermodynamic properties and structural phase transition of NbN under high pressure are investigated by means of the pseudopotential plane- waves method, in addition to the effect of metallic bonding on its hardness. Three candidate structures are chosen to investigate NbN, namely, rocksalt (NaCl), NiAs and WC types. On the basis of the third- order Birch- Murnaghan equation of states, the transition pressure Pt (Pt = 200.64GPa) between the WC phase and the NaCl phase of NbN is predicted for the first time. Elastic constants, formation enthalpies, shear modulus, Youngs modulus, and Poissons ratio of NbN are derived. The calculated results are found to be in good agreement with the available experimental data and theoretical values. According to the quasi-harmonic Debye model, the Debye temperature under high pressure is derived from the average sound velocity. Moreover, the effect of metallic bonding on the hardness of NbN is investigated and the hardness shows a gradual decrease rather than increase under compression. This is a quantitative investigation on the structural and thermodynamic properties of NbN, and it still awaits experimental confirmation. Copyright (C) EPLA, 2010

Pressure-induced structural transition of OsN2 and effect of metallic bonding on its hardness

Using first-principles calculations, the elastic constant, structural phase transition and effect of metallic bonding on the hardness of OsN2 under high pressure are investigated by means of the pseudopotential plane-waves method. Five candidate structures are chosen to investigate for OsN2, namely, the pyrite, CoSb2-type, marcasite, simple hexagonal and tetragonal structures. A comparison among the formation energies of OsN2 explains the synthesis of OsN2 marcasite under high pressure. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure Pt (Pt=223?GPa) between the marcasite and simple tetragonal phase is determinated. Elastic constants, shear modulus, Youngs modulus, Poissons ratio and Debye temperature are derived. The calculated values are, generally speaking, in good agreement with experiments and other theoretical calculations. Our calculation indicates that the N-N bond length is one determinative factor for the ultrahigh bulk moduli of the heavy-transition-metal dinitrides. Moreover, based on Mulliken overlap population analysis in first-principles technique, a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bonding is presented. The effect of metallic bonding on the hardness of OsN2 is investigated and the hardness shows a gradual decrease rather than increase under compression, which is different from diamond. This is a quantitative investigation on the structural properties of OsN2, and it still awaits experimental confirmation.

Structural and Relative Stabilities, Electronic Properties, and Hardness of Iron Tetraborides from First Prinicples

First-principles calculations were carried out to investigate the structure, phase stability, electronic property, and roles of metallicity in the hardness for recently synthesized FeB4 with various different structures. Our calculation indicates that the orthorhombic phase with Pnnm symmetry is the most energetically stable one. The other four new dynamically stable phases belong to space groups monoclinic C2/m, orthorhombic Pmmn, trigonal R3̅m, and hexagonal P63/mmc. Their mechanical and thermodynamic stabilities are verified by calculating elastic constants, formation enthalpies, and phonon dispersions. We found that all phases are stabilized further under pressure. Above the pressure of about 50 GPa, the formation enthalpy of Pmmn is almost equal to that of P63/mmc phase. The analysis on density of states not only demonstrates that formation of strong covalent bonding in these compounds contributes greatly to their stabilities but also that they all exhibit metallic behavior which does not relate to the approach used. By considering metallic contributions, the estimated Vickers hardness values based on the semiempirical model show that the OsB4-structured FeB4, with a hardness of 48.1 GPa, well exceeding the limitation of superhardness (40 GPa), is more hard than the most stable phase. The others are predicted to be potential hard materials. Moreover, the atomic configuration and strong B-B covalent bonds are found to play important roles in the hardness of materials.

Phase stability, mechanical properties, hardness, and possible reactive routing of chromium triboride from first-principle investigations

The first-principles calculations are employed to provide a fundamental understanding of the structural features and relative stability, mechanical and electronic properties, and possible reactive route for chromium triboride. The predicted new phase of CrB3 belongs to the rhombohedral phase with R-3m symmetry and it transforms into a hexagonal phase with P-6m2 symmetry at 64 GPa. The mechanical and thermodynamic stabilities of CrB3 are verified by the calculated elastic constants and formation enthalpies. Also, the full phonon dispersion calculations confirm the dynamic stability of predicted CrB3. Considering the role of metallic contributions, the calculated hardness values from our semiempirical method for rhombohedral and hexagonal phases are 23.8 GPa and 22.1 GPa, respectively. In addition, the large shear moduli, Youngs moduli, low Poissons ratios, and small B∕G ratios indicate that they are potential hard materials. Relative enthalpy calculations with respect to possible constituents are also investigated to assess the prospects for phase formation and an attempt at high-pressure synthesis is suggested to obtain chromium triboride.

First-principles studies of the structural, electronic and optical properties of Zn1−xCdxS and ZnS1−ySey alloys

Using the density functional theory calculations, we studied the structural, electronic and optical properties of zincblende Zn1−xCdxS and ZnS1−ySey alloys. Calculated structures and band gaps of these alloys are in good agreement with available experimental and other theoretical values. Present results show that the prominent peaks of the dielectric functions and absorption coefficients have a slight red shift and the amplitudes become larger with the increasing concentration of Cd and Se. Moreover, present findings predict that Zn1−xCdxS and ZnS1−ySey alloys are promising for solar cells and photoconductor and electroluminescent devices due to their high absorption of solar radiations and photoconductivity in the energy region from visible light to ultraviolet.

Effect of pressure on structural phase transition and elastic properties in NiSi

A systematic study of the structural phase transition, elastic, and electronic properties for nickel silicide (NiSi) under high pressure has been carried out by using the pseudopotential plane-wave density functional theory. Among them, the mechanism of the high pressure structural phase transition has been studied in a detailed manner. Our results show that the structural phase transition pressure from MnP- to CuTi-structure in NiSi is 14.6 GPa, which is in good agreement with other theoretical report. We have also calculated the elastic constants, bulk modulus, shear modulus, Youngs modulus, Poissons coefficients, and elastic anisotropy factors of MnP-type NiSi under high pressure. We find that the pressure has an important influence on the elastic modulus. Moreover, on the basis of the Mulliken overlap population analysis, the hardness H ν (H ν=11.3 GPa) of the MnP-type NiSi has been obtained, which is compatible with the experimental value.

STRUCTURAL, ELASTIC AND MECHANICAL PROPERTIES OF TMN (TM = Ti,V,Cr): A DFT STUDY

Mechanical properties and the effect of metallic bonding on the hardness of transition-metal nitrides (TiN, VN and CrN) compounds are studied using the first-principles calculation. Present results show that these transition-metal nitrides are mechanically stable and the VN and CrN are ductile, whereas TiN is predicted to be brittle. Moreover, it is found that the high hardness of TiN, VN and CrN exhibits a remarkable decrease with transition-metal changed from Ti to Cr, and the metallic d–d interactions play important roles on determining the hardness of transition-metal nitrides.