Pressure-induced structural phase transition in transition metal carbides TMC (TM = Ru,Rh, Pd,Os, Ir,Pt): a DFT study

First-principles calculations based on density functional theory was performed to analyse the structural stability of transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt). It is observed that zinc-blende phase is the most stable one for these carbides. Pressure-induced structural phase transition from zinc blende to NiAs phase is predicted at the pressures of 248.5 GPa, 127 GPa and 142 GPa for OsC, IrC and PtC, respectively. The electronic structure reveals that RuC exhibits a semiconducting behaviour with an energy gap of 0.7056 eV. The high bulk modulus values of these carbides indicate that these metal carbides are super hard materials. The high B/G value predicts that the carbides are ductile in their most stable phase.     

Structural,electronic and mechanical properties of alkaline earth metal oxides MO (M=Be,Mg, Ca,Sr, Ba)

The structural, electronic and mechanical properties of alkaline earth metal oxides MO (M=Be, Mg, Ca, Sr, Ba) in the cubic (B1, B2 and B3) phases and in the wurtzite (B4) phase are investigated using density functional theory calculations as implemented in VASP code. The lattice constants, cohesive energy, bulk modulus, band structures and the density of states are computed. The calculated lattice parameters are in good agreement with the experimental and the other available theoretical results. Electronic structure reveals that all the five alkaline earth metal oxides exhibit semiconducting behavior at zero pressure. The estimated band gaps for the stable wurtzite phase of BeO is 7.2 eV and for the stable cubic NaCl phases of MgO, CaO, SrO and BaO are 4.436 eV, 4.166 eV, 4.013 eV, and 2.274 eV respectively. A pressure induced structural phase transition occurs from wurtzite (B4) to NaCl (B1) phase in BeO at 112.1 GPa and from NaCl (B1) to CsCl (B2) phase in MgO at 514.9 GPa, in CaO at 61.3 GPa, in SrO at 42 GPa and in BaO at 14.5 GPa. The elastic constants are computed at zero and elevated pressures for the B4 and B1 phases for BeO and for the B1 and B2 phases in the case of the other oxides in order to investigate their mechanical stability, anisotropy and hardness. The sound velocities and the Debye temperatures are calculated for all the oxides using the computed elastic constants.     

Structural,mechanical, and electronic properties of P3m1-BCN

The mechanical and electronic properties of P3m1-BCN have been studied by using first principles calculations. The anisotropy studies of Young's modulus, shear modulus and Poisson's ratio show that P3m1-BCN exhibits a large anisotropy. Electronic structure study shows that P3m1-BCN is an indirect semiconductor with band gap of 4.10 eV. Unusually, the band gap of P3m1-BCN increase with increasing pressure.     

Trends in stability, elastic and electronic properties of cubic Rh, Ir, Pd and Pt carbides depending on carbon content: MC versus M4C from first-principles calculations

First principles FLAPW-GGA calculations have been performed to understand the peculiarities of stability, elastic, electronic properties and chemical bonding for cubic carbides of four noble metals M=Rh, Pd, Ir and Pt depending on carbon stoichiometry: MC versus M₄C. Our main findings are as follows: (i) in contrast to mono-carbides MC with positive formation energies E_form>0, carbon-deficient sub-carbides M₄C are stable (E_form<0), thus carbon stoichiometry is one of the major factors determining successful synthesis of these materials, and (ii) as distinct from the majority of other 3d-5d metals (including Pd and Pt examined here), an unusual effect of Rh and Ir “metallization” and the increasing of ductility for these metals owing to the introduction of carbon has been established.     

First principles study of structural stability,electronic structure and mechanical properties of ReN and TcN

The crystal structure, structural stability, electronic and mechanical properties of ReN and TcN are investigated using first principles calculations. We have considered five different crystal structures: NaCl, zinc blende (ZB), NiAs, tungsten carbide (WC) and wurtzite (WZ). Among these ZB phase is found to be the lowest energy phase for ReN and TcN at normal pressure. Pressure induced structural phase transitions from ZB to WZ phase at 214 GPa in ReN and ZB to NiAs phase at 171 GPa in TcN are predicted. The electronic structure reveals that both ReN and TcN are metallic in nature. The computed elastic constants indicate that both the nitrides are mechanically stable. As ReN in NiAs phase has high bulk and shear moduli and low Poisson's ratio, it is found to be a potential ultra incompressible super hard material.     

Structural phase stability,electronic structure and mechanical properties of alkali metal hydrides AMH4 (A=Li,Na; M=B,AL)

The structural stability of Alkali metal hydrides AMH₄ (A=Li, Na; M=B, Al) is analyzed among the various crystal structures, namely hexagonal (P6₃mc), tetragonal (P4₂/nmc), tetragonal (P-42₁c), tetragonal (I4₁/a), orthorhombic (Pnma) and monoclinic (P2₁/c). It is observed that, orthorhombic (Pnma) phase is the most stable structure for LiBH₄, monoclinic (P2₁/c) for LiAlH₄, tetragonal (P4₂/nmc) for NaBH₄ and tetragonal (I4₁/a) for NaAlH₄ at normal pressure. Pressure induced structural phase transitions are observed in LiBH₄, LiAlH₄, NaBH₄ and NaAlH₄ at the pressures of 4 GPa, 36.1 GPa, 26.5 GPa and 46 GPa respectively. The electronic structure reveals that these metal hydrides are wide band gap insulators. The calculated elastic constants indicate that these metal hydrides are mechanically stable at normal pressure.     

Structural, electronic properties and stability of tungsten mono- and semi-carbides: A first principles investigation

First principles calculations have been performed with the purpose to understand the comparative peculiarities of the structural, electronic properties and stability for all phases formed in the tungsten-carbon system: hexagonal and cubic mono-carbides WC and four polymorphs (α, β, γ and ε) of semi-carbide W₂C. All calculations were performed by means of the full-potential linearized augmented plane wave method (FLAPW). The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) formalism was used for the exchange and correlation energy functional. The geometries of all WC and W₂C phases were optimized and their structural parameters and theoretical density were established. Besides, we have evaluated the formation energies (E_form) of all the tungsten carbides. Based on our estimations we can arrange all investigated W-C phases depending on their stability in the following sequence: h-WC>ε-W₂C>β-W₂C>γ-W₂C>α-W₂C>c-WC. Here three carbides (h-WC, ε-W₂C and β-W₂C) are stable (E_form<0), γ-W₂C belongs to metastable systems (E_form∼0), whereas α-W₂C and c-WC appear to be unstable (E_form>0). Moreover, band structures, total and partial densities of states were obtained and analyzed systematically for all W-C phases in comparison with other available theoretical and experimental data.     

Structural,electronic and magnetic properties of the (001), (110) and (111) surfaces of rocksalt sodium sulfide: A first-principles study

First principles study of the structural, electronic and magnetic properties of the (111), (110) and (001) surfaces of rocksalt sodium sulfide (rs-NaS) are reported. The results show that the bulk half-metallicity of this compound is well preserved on the surfaces considered here except for Na-terminated (111) surface. The spin-flip gap at the S-terminated (111), (001) and (110) surfaces are close to the bulk value. Using ab-initio atomistic thermodynamics, we calculate the surface energies as a function of chemical potential to find the most stable surface. We find that the Na-terminated (111) surface is the most stable one over the whole allowed range of chemical potential, while the surface energies of the (001) and (110) surfaces approach the most stable surface energy at the sulfur rich environment. We have also calculated the interlayer exchange interaction in bulk and Na-terminated (111) surface by classical Heisenberg model and we found that the surface effects do not change these kinds of interactions significantly.     

Structural, high pressure and elastic properties of transition metal monocarbides: A FP-LAPW study

The structural, elastic and thermal properties of four transition metal monocarbides ScC, YC (group III), VC and NbC (group V) have been investigated using full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) both at ambient and high pressure. We predict a B₁ to B₂ structural phase transition at 127.8 and 80.4 GPa for ScC and YC along with the volume collapse percentage of 7.6 and 8.4%, respectively. No phase transition is observed in case of VC and NbC up to pressure 400 and 360 GPa, respectively. The ground state properties such as equilibrium lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′) are determined and compared with available data. We have computed the elastic moduli and Debye temperature and report their variation as a function of pressure.     

A comparative study on electronic and structural properties of transition metal monosilicides, CrSi(B20-type), RhSi(B20-type), RhSi(B31-type) and RhSi(B2-type)

Transition metal based monosilicide compounds (CrSi and RhSi) have been investigated theoretically from ab initio calculations. The structural and electronic band calculations of CrSi and different phases of RhSi crystals show that the metallic property and hypothetically constructed structures of RhSi(Pnma) under different pressures from 0 GPa to 75 GPa show a certain difference only along Γ–Z directions of the high symmetry points of first Brillouin zone. The character of the bands around fermi level was determined by partial density of state calculations.     

Single crystal growth,electronic structure and optical properties of Cs2HgBr4

We report on successful synthesis of high-quality single crystal of cesium mercury tetrabromide, Cs₂HgBr₄, by using the vertical Bridgman–Stockbarger method as well as on studies of its electronic structure. For the Cs₂HgBr₄ crystal, we have recorded X-ray photoelectron spectra for both pristine and Ar⁺ ion-bombarded surfaces. Our data indicate that the Cs₂HgBr₄ single crystal surface is rather sensitive with respect to Ar⁺ ion-bombardment. In particular, such a treatment of the Cs₂HgBr₄ single crystal surface alters its elemental stoichiometry. To explore peculiarities of the energy distribution of total and partial densities of states within the valence band and the conduction band of Cs₂HgBr₄, we have made band-structure calculations based on density functional theory (DFT) employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The APW+lo calculations allow for concluding that the Br 4p states make the major contributions in the upper portion of the valence band, while its lower portion is dominated by contributors of the Hg 5d and Cs 5p states. Further, the main contributors to the bottom of the conduction band of Cs₂HgBr₄ are the unoccupied Br p and Hg s states. In addition, main optical characteristics of Cs₂HgBr₄ such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity have been explored from the first-principles band-structure calculations.     

Structural,electronic and optical properties of LiBeP in its normal and high pressure phases

An investigation on the structural stabilities, electronic and optical properties of LiBeP under high pressure was conducted using the all-electron density functional theory within the local density approximation. Our results show that the sequence of the pressure induced phase transition of LiBeP is the Cu₂Sb-type structure (P4/nmm), the MgSrSi-type structure (Pnma) and the LiGaGe-type structure (P6₃mc). The first transition (P4/nmm to Pnma) takes place at 2.95 GPa and the second (Pnma to P6₃mc) at 6.65 GPa. In the three phases, the bandgap is indirect and the valence band maximum is at the zone center. With increasing pressure LiBeP in the LiGaGe structure becomes a direct gap semiconductor at 19.75 GPa. The assignments of the structures in the optical spectra and the band structure transitions are discussed. The mean value of the optical dielectric constant for the Cu₂Sb phase is smaller than that for the MgSrSi and the LiGaGe ones. This compound has a positive uniaxial anisotropy in the LiGaGe structure. The absorption coefficient along the z   direction, _αzz${\alpha }_{\mathit{zz}}$, for the MgSrSi structure is higher than that in the other two structures in the visible regime.     

Local structure and electronic properties of BaTaO2N with perovskite-type structure

First-principles calculation based on density-functional theory in the pseudo-potential approach have been performed for the total energy and crystal structure of BaTaO₂N. The calculations indicate a random occupation of the anionic positions by O and N in a cubic structure, in agreement with neutron diffraction measurements and infrared spectra. The local symmetry in the crystal is broken, maintaining a space group Pm3?m, as used in structure refinement, which represents only the statistically averaged result. The calculations also indicate displacive disordering in the crystal. The average Ta-N distance is smaller (2.003 Å), while the average Ta-O distance becomes larger (2.089 Å). The local relaxation of the atoms has an influence on the electronic structure, especially on the energy gap. BaTaO₂N is calculated to be a semiconductor with an energy gap of about 0.5 eV. The upper part of the valence band is dominated by N 2p states, while O 2p states are mainly in the lower part. The conduction band is dominated by Ta 5d states.     

Structural stability and electronic properties of Co2N, Rh2N and Ir2N

The structural stability and electronic properties of Co₂N, Rh₂N and Ir₂N were studied by using the first principles based on the density functional theory. Two structures were considered for each nitride, orthorhombic Pnnm phase and cubic Pa3¯ phase. The results show that they are all mechanically stable. Co₂N in both phases are thermodynamically stable due to the negative formation energy, while the remaining two compounds are thermodynamically unstable. The calculated properties show that they are all metallic and non-magnetic. Ir₂N at Pnnm phase is a potentially hard material. The bonding behavior is analyzed.     

Structural transition of Ti3SiC2 under high hydrostatic pressure: A first-principles study

We theoretically studied the phase transformation, electronic and elastic properties of Ti₃SiC₂ ceramic by using the pseudopotential plane-wave method within the density functional theory. Our results demonstrate that there exists a structural phase transition from α‐Ti₃SiC₂ to β‐Ti₃SiC₂ under pressure up to 384 GPa, and α‐Ti₃SiC₂ is the most stable phase at zero pressure. The calculated electronic band structure and density of states reveal the metallic behavior for the polymorphs of Ti₃SiC₂. The mechanical stability of α‐Ti₃SiC₂ at zero pressure is confirmed by the elastic constants, and is analyzed in terms of electronic level. By analyzing the ratio between bulk and shear moduli, we conclude that α‐Ti₃SiC₂ is brittle in nature.     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

Ab initio electronic, dynamic, and thermodynamic properties of isotopic lithium hydrides (LiH, LiD, LiT, LiH, LiD, and LiT)

The structural, dielectric, lattice-dynamical, and thermodynamical properties of isotopic lithium hydrides (⁶LiH, ⁶LiD, ⁶LiT, ⁷LiH, ⁷LiD, and ⁷LiT) were investigated within density-functional theory. The atomic structure was fully relaxed and the structural parameters were found to differ by less than 2% from the experimental data. The associated electronic band structure and density of states were also presented. A linear-response approach to the density-functional perturbation theory was employed to work out the Born effective charges, dielectric tensors and phonon frequencies, and thermodynamic properties. The compounds with the heavier Li isotope or H isotope have the lower phonon frequencies; ⁶LiT is more stable than ⁷LiT, ⁶LiD, ⁷LiD, ⁶LiH, and ⁷LiH in the temperature range 0-2700 K. These properties of LiT were predicted for the first time. The results were discussed in terms of the isotope effects on phonon dispersion curves and thermodynamic properties.     

Structural, electronic and phonon properties’ investigation of YP and YAs compounds

We have studied the structural, electronic and phonon properties of the YP and YAs compounds in NaCl(B1) and CsCl(B2) structures using the density functional theory within the generalized gradient approximation (GGA). The calculated lattice constants, static bulk modulus, first-order pressure derivative of the bulk modulus and transition pressure are reported and compared with previous calculations. We have carried out the calculations of band structure and density of states (DOS) for YP and YAs. Then, a linear-response approach to the density-functional theory is used to derive the phonon frequencies and DOS in both B1 and B2 structures.     

DFT study on the electronic structure and chemical state of Americium in an (Am,U) mixed oxide

We investigated the electronic state of an (Am,U) mixed oxide with the fluorite structure using the all-electron full potential linear augmented plane wave method and compared it with those of Am₂O₃, AmO₂, UO₂, and La_0.5U_0.5O₂. The valence of Am in the mixed oxide was close to that of Am₂O₃ and the valence of U in the mixed oxide was pentavalent. The electronic structure of AmO₂ was different from that of Am₂O₃, particularly just above the Fermi level. In addition, the electronic states of Am and U in the mixed oxide were similar to those of trivalent Am and pentavalent U oxides. These electronic states reflected the high oxygen potential of AmO₂ and the heightened oxygen potential resulting from the addition of Am to UO₂ and also suggested the occurrence of charge transfer from Am to U in the solid solution process.     

Structural and elastic properties of the filled tetrahedral semiconductors LiZnX (X=N, P, and As)

We report on first-principles study of the structural and elastic properties of the Nowotny-Juza filled tetrahedral compounds LiZnX (X=N, P, As) using the full-potential linearized augmented plane wave method within the local density approximations. Our results indicate that the energetically favourable α-LiZnX materials are slightly softer than their binary analogous GaX and the sound speeds are quantitatively similar for LiZnAs and GaAs.