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The study of stable half-metallic ferromagnetic materials is important from various fundamental and application points of view in condensed matter Physics. Structural phase stability, electronic structure, mechanical and magnetic properties of Fe-based quaternary Heusler alloys XX′YZ (X = Co, Ni; X′ = Fe; Y = Ti; Z = Si, Ge, As) for three different phases namely α, β and γ phases of LiMgPdSn crystal structure have been studied by density functional theory with generalized gradient approximation formulated by Perdew, Burke and Ernzerhof (GGA-PBE) and the Hubbard formalism (GGA-PBE + U). This work aims to identify the ferromagnetic and half-metallic properties of XX′YZ (X = Co, Ni, X′ = Fe; Y = Ti; Z = Si, Ge, As) quaternary Heusler alloys. The predicted phase stability shows that α-phase is found to be the lowest energy phase at ambient pressure. A pressure-induced structural phase transition is observed in CoFeTiSi, CoFeTiGe, CoFeTiAs, NiFeTiSi, NiFeTiGe and NiFeTiAs at the pressures of 151.6 GPa, 33.7 GPa, 76.4 GPa, 85.3 GPa, 87.7 GPa and 96.5 GPa respectively. The electronic structure reveals that these materials are half metals at normal pressure whereas metals at high pressure. The investigation of electronic structure and magnetic properties are performed to reveal the underlying mechanism of half metallicity. The spin polarized calculations concede that these quaternary Heusler compounds may exhibit the potential candidate in spintronics application. The magnetic moments for these quaternary Heusler alloys in all the three different phases (α, β and γ) are estimated. [ABSTRACT FROM AUTHOR]

The search for stable half-metallic ferromagnetic materials remains a high priority in condensed matter Physics. Ab initio calculations are performed using density functional theory to analyze the structural phase stability, electronic structure, mechanical and magnetic properties of Mn based half Heusler alloys XYZ (X = Ir, Pt, Au; Y = Mn; Z = Sn, Sb) for three different phases namely α, β and γ phases of C1 b crystal structure. This work aims to identify the ferromagnetic and half-metallic behavior of XYZ (X = Ir, Pt, Au; Y = Mn; Z = Sn, Sb) half Heusler alloys. To accomplish this, density functional theory (DFT) with generalized gradient approximation formulated by Perdew, Burke and Ernzerhof (GGA-PBE) and the Hubbard formalism (GGA-PBE + U) are used to describe the strong correlations present in these alloys. Among the considered phases, α-phase is found to be the lowest energy phase for IrMnSn, IrMnSb, PtMnSn, PtMnSb, AuMnSn and AuMnSb at normal pressure. A pressure-induced structural phase transition is observed in IrMnSn, IrMnSb, PtMnSn, PtMnSb, AuMnSn and AuMnSb at the pressures of 62.1 GPa, 47.8 GPa, 26.1 GPa, 25.3 GPa, 23.7 GPa and 14.0 GPa respectively. The electronic structure reveals that these materials are metals at normal pressure whereas half metals at high pressure. The magnetic moments for these half Heusler alloys in all the three different phases (α, β and γ) are estimated. [ABSTRACT FROM AUTHOR]

The structural stability of Alkali metal hydrides AMH 4 (A=Li, Na; M=B, Al) is analyzed among the various crystal structures, namely hexagonal (P6 3 mc), tetragonal (P4 2 /nmc), tetragonal (P-42 1 c), tetragonal (I4 1 /a), orthorhombic (Pnma) and monoclinic (P2 1 /c). It is observed that, orthorhombic (Pnma) phase is the most stable structure for LiBH 4 , monoclinic (P2 1 /c) for LiAlH 4 , tetragonal (P4 2 /nmc) for NaBH 4 and tetragonal (I4 1 /a) for NaAlH 4 at normal pressure. Pressure induced structural phase transitions are observed in LiBH 4 , LiAlH 4 , NaBH 4 and NaAlH 4 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. [ABSTRACT FROM AUTHOR]

The structural, electronic, elastic and magnetic properties of gadolinium and its hydrides GdH x ( x =1, 2, 3) are investigated by using Vienna ab-initio simulation package with the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) plus a Hubbard parameter (GGA-PBE+ U ) in order to include the strong Coulomb correlation between localized Gd 4f electrons. At ambient pressure all the hydrides are stable in the ferromagnetic state. The calculated lattice parameters are in good agreement with the experimental results. The bulk modulus is found to decrease with the increase in the hydrogen content for the gadolinium hydrides. A pressure-induced structural phase transition is predicted to occur from cubic to hexagonal phase in GdH and GdH 2 and from hexagonal to cubic phase in GdH 3 . The electronic structure reveals that mono and di-hydrides are metallic, whereas trihydride is half-metallic at normal pressure. On further increasing the pressure, a half-metallic to metallic transition is also observed in GdH 3 . The calculated magnetic moment values of GdH x ( x =1, 2, 3) are in accord with the experimental values. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

First principles calculations are performed to investigate the structural stability, electronic and mechanical properties of ZnN and CdN for various structures: NaCl, CsCl, zinc blende, and NiAs. The local density and generalized gradient approximations are used for exchange and correlation effects. Our calculations indicate that zinc blende (ZB) phase is the lowest energy phase for ZnN while it is the NaCl phase for CdN at normal pressure. A pressure induced structural phase transition from zinc blende to NiAs phase is predicted in ZnN, whereas NaCl to zinc blende phase and then zinc blende to NiAs phase is observed in CdN. The ground state properties like lattice parameters, bulk modulus and cohesive energy are calculated. Electronic structure reveals that both ZnN and CdN are metallic. The computed elastic constants show that these nitrides are mechanically stable in their most stable phase. The shear modulus, Young’s modulus, Poisson’s ratio and Debye temperature are also calculated. The low bulk modulus value indicates that ZnN and CdN are more compressible compared to other transition metal nitrides. [ABSTRACT FROM AUTHOR]

The structural stability, electronic structure and mechanical properties of platinum group metal nitrides PGMN (PGM = Os, Ir, Pt) are investigated using the first principles calculation. The structural stability is analyzed among six different crystal structures namely, NaCl, CsCl, zinc blende, NiAs, WC and wurtzite structures. The zinc blende structure is found to be the most stable one for all the three nitrides at normal pressure. A structural phase transition from zinc blende to the NiAs structure is predicted at the pressures of 71 GPa, 41 GPa and 24 GPa for OsN, IrN and PtN respectively. The electronic structure reveals that these nitrides are metallic. The high bulk modulus values of OsN, IrN and PtN indicate that these metal nitrides are super hard materials. Moreover, a positive value of calculated Cauchy pressure and a high B/G value predict that these materials are ductile in nature. The Debye temperature values are also reported for these nitrides. [ABSTRACT FROM AUTHOR]

Ab initio calculations are performed to investigate the structural stability, electronic structure and elastic properties of noble metal sub-nitrides M2N (M = Ru, Rh, Pd). The metal nitrides, Ru2N and Rh2N are found to be stable in anti-fluorite structure, whereas Pd2N is stable in tetragonal structure. In Ru2N and Rh2N, structural phase transition is predicted from antifluorite to orthorhombic phase whereas tetragonal to antifluorite and then antifluorite to pyrite phase transitions are predicted in Pd2N under high pressure. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition. The Debye temperature values are reported for all the phases of these nitrides. The high bulk modulus indicates that these materials are super hard materials. They are ductile in nature at normal and high pressures. The bonding nature of these materials is found to be a mixture of covalent, metallic and ionic attribution at ambient condition. [ABSTRACT FROM AUTHOR]

Abstract: Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104GPa, 50.5GPa and 56GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition. [Copyright &y& Elsevier]

A theoretical investigation of the structural, electronic and elastic properties of the transition metal monoxides Zinc monoxide (ZnO) and Cadmium monoxide (CdO) in the cubic B1 (rock-salt) B2 (CsCl) and B3 (zinc blende) phases are performed using the first-principles calculations. The optimized lattice parameters and their bulk modulus values are found to be in accord with the available experimental and other theoretical values. To understand the electronic properties of ZnO and CdO, their electronic band structures are analyzed. A pressure induced structural phase transition occurs from B3 to B1 at a pressure of 11.2 GPa and from B1 to B2 at even higher pressure of 243.3 GPa occurs in ZnO; whereas the transition is from B1 to B2 phase at a pressure of 88.5 GPa in CdO. The elastic constants are computed to investigate the mechanical stability at normal and elevated pressures. The pressure dependence of the various moduli, Poisson's ratio, anisotropy and the hardness of ZnO and CdO are also analyzed. [ABSTRACT FROM AUTHOR]

Abstract: The structural, electronic, elastic and magnetic properties of three rare earth nitrides (RENs: RE=Gd, Dy, Ho) are investigated by first principles calculation using Vienna ab-initio simulation code (VASP). At ambient pressure all the three nitrides are stable in the ferromagnetic state (FM) with cubic NaCl(B1) structure. The calculated lattice parameters are in good agreement with experimental and other theoretical values. Electronic structure reveals that these materials are half metallic ferromagnets at normal pressure. A pressure induced structural phase transition from NaCl to wurtzite phase in GdN at 18.2GPa and from NaCl to CsCl phase in DyN at 104GPa and HoN at 138GPa is also predicted. The half metallicity has been retained in GdN, DyN and HoN even after the phase transition but it vanishes at 73.18GPa, 118GPa and 141GPa respectively. Ferromagnetism is quenched in all the three nitrides at a pressure of 252GPa (GdN), 242GPa (DyN) and 236.7GPa (HoN). [Copyright &y& Elsevier]

Highlights: [•] Electronic, structural and elastic properties of group IB nitrides are investigated. [•] A pressure induced structural phase transition is predicted under high pressure. [•] Electronic structure reveals that these materials exhibit metallic behavior. [•] Computed elastic moduli obey traditional mechanical stability condition. [•] The Debye temperature values are computed for Group IB metal mono and di-nitrides. [ABSTRACT FROM AUTHOR]

Abstract: The electronic, structural, mechanical and superconducting properties of group VB mononitrides are investigated by means of first principles calculation based on density functional theory with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and theoretical results. Among the three crystallographic structures that have been investigated, the hexagonal WC phase is found to more stable than the cubic ones. Under high pressure, a series of structural phase transition from WC → NaCl → CsCl phase is also predicted in VN, NbN and TaN. The calculated elastic constants indicate that all the three nitrides are mechanically stable at ambient pressure. The estimated Zener ratio and linear compressibility coefficients K c /K a reveals that these materials exhibit elastic anisotropy. The estimated superconducting transition temperature (T c ) values as a function of pressure for VN, NbN and TaN are 35.5, 37.5 and 30.5 K respectively. [Copyright &y& Elsevier]

Abstract: First-principles calculation was performed using tight-binding LMTO method with local density approximation (LDA) and atomic sphere approximation (ASA) to understand the electronic properties of rhenium nitride. The equilibrium geometries, the electronic band structure, the total and partial DOS are obtained under various pressures and are analyzed in comparison with the available experimental data. The most stable structure of ReN is NiAs like structure. Our results indicate that ReN can be used as a super-hard conductor. We estimated the average electron–phonon coupling constant to be 1.65 and superconducting transition temperature (T c) is 5.1K. The T c value increases with the increase in pressure. [Copyright &y& Elsevier]

Abstract: First principles calculation was performed using tight-binding LMTO method with local density approximation (LDA) and atomic sphere approximation (ASA) to understand the electronic properties of chromium nitride. The equilibrium geometries, the magnetic moment, the electronic band structure, the total and partial DOS are obtained under various pressures and are analyzed in comparison with the available experimental data. The most stable structure of CrN is NaCl structure in the FM state. A pressure-induced second order magnetic phase transition from ferromagnetic (FM) to non-magnetic (NM) at very high pressure of 0.5549Mbar is predicted. Our results indicate that CrN can be used as a hydrogen storage material. [Copyright &y& Elsevier]

Abstract The electronic structure, magnetic and mechanical properties of XMgC (X = Li, Na, K, Rb) in half – Heusler (C1 b) type structure are computed for various phases such as α, β and γ phases using first principles calculation based on density functional theory with generalised gradient approximation as the exchange correlation function and HSE06 hybrid function. The β phase is found to be the most stable one for these carbides at normal pressure. A structural phase transition from β to α phase in NaMgC and β to γ phase in RbMgC is predicted at the pressures of 19 GPa and 14 GPa respectively. The electronic structure calculation with HSE06 hybrid function predicts that LiMgC, NaMgC, KMgC and RbMgC are half metallic materials with energy gap of 1.855 eV for LiMgC in spin down stare and 2.172 eV, 2.101 eV, 1.739 eV in the spin up state for NaMgC, KMgC and RbMgC respectively. The calculated elastic constants satisfy the stability criteria and hence these carbides are mechanically stable in their most stable phase. The bulk modulus, shear modulus, Young's modulus and Poisson's ratio are also calculated. Highlights • Structural stability of XMgC (X = Li,Na,K,Rb) is analysed. • These carbides exhibit half-metallic ferromagnetism. • Structural phase transition is observed at high pressure. • Ferromagnetic to non-magnetic phase transition is observed. • Phonon dispersion curve predicts that LiMgC and NaMgC are dynamically stable. [ABSTRACT FROM AUTHOR]

Abstract The search for hard refractory materials has recently been attractive in the field of material science. The structural stability, electronic structure, and mechanical properties of noble metal carbides M 2 C and MC 2 (M = Os, Ir, Pt) are investigated in tetragonal (P 4/ mbm), fluorite/antifluorite (Fm3m), orthorhombic (Pnnm), pyrite (Pa-3) and hexagonal (P 6/ mmm) phases for their potential ability and use as hard materials, by the first principles calculation based on density functional theory. It is found that M 2 C are most stable in anti -fluorite structure, while MC 2 are stable in hexagonal structure. In Os 2 C and Pt 2 C, structural phase transition is predicted from antifluorite to pyrite whereas antifluorite to hexagonal in Ir 2 C and hexagonal to tetragonal in OsC 2 and PtC 2 under high pressure. Born-Huang criteria for mechanical stability are satisfied by the elastic constants of M 2 C and MC 2. The bulk modulus of these materials is found to be high, which indicates that these materials are super hard materials. Highlights • Metal carbides M 2 C are stable in anti -fluorite structure. • Metal carbides MC 2 are stable in hexagonal structure. • Structural phase transition is observed at high pressure. • Electronic structure indicates that these carbides are metallic. • Mechanical properties indicate that these metal carbides are superhard materials. [ABSTRACT FROM AUTHOR]

The structural stability of rare earth nitrides REN (RE=Ce,Pr,Nd) is investigated among three cubic structures, namely, NaCl (B1), CsCl(B2) and zinc blende (B3). It is found that NaCl structure is the most stable structure for all the three nitrides. On increasing the pressure, structural phase transition from NaCl (B1) to CsCl (B2) phase is predicted in CeN and NdN at the pressures of 88 GPa and 36.5 GPa while NaCl (B1) to zinc blende (B3) phase transition is observed in PrN at the pressure of 68 GPa. At normal pressure, all the three nitrides are stable in the ferromagnetic state (FM) with cubic NaCl (B1) structure. The calculated lattice parameters and bulk modulus values are in good agreement with experimental and other theoretical values. Electronic structure reveals that PrN and NdN are half metallic while CeN is metallic at normal pressure. Ferromagnetism is quenched in CeN and PrN at the pressures of 152 GPa and 121 GPa respectively. The positive values of elastic constants indicate that all the three nitrides are mechanically stable in NaCl Phase. It is found that all these nitrides are ductile and anisotropic in nature. [ABSTRACT FROM AUTHOR]

The structural, electronic, mechanical and superconducting properties of rhenium carbide (ReC) and technetium carbide (TcC) are investigated using first principles calculations based on density functional theory (DFT). The computed ground state properties like equilibrium lattice constants and cell volume are in good agreement with the available experimental and theoretical data. It is observed that tungsten carbide phase (WC) is the most stable phase for ReC and TcC at normal pressure. Pressure induced structural phase transitions are observed from tungsten carbide to zinc blende phase and then zinc blende to wurtzite phase in rhenium carbide while tungsten carbide to zinc blende phase and then zinc blende to nickel arsenide phase in technetium carbide. Electronic structure reveals that these materials are metallic at ambient condition. The high bulk modulus (B) value of WC phase of both ReC (438 GPa) and TcC (368 GPa) suggests that they are ultra hard and incompressible materials. Also, the superconducting transition temperature is estimated for ReC and TcC at normal pressure. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The structural, electronic, mechanical and superconducting properties of tungsten carbide (WC) and tungsten nitride (WN) are investigated using first principles calculations based on density functional theory (DFT). The computed ground state properties, such as equilibrium lattice constant and cell volume, are in good agreement with the available experimental data. A pressure induced structural phase transition is observed in both tungsten carbide and nitride, from a tungsten carbide phase (WC) to a zinc blende phase (ZB), and from a zinc blende phase (ZB) to a wurtzite phase (WZ). The electronic structure reveals that these materials are metallic at ambient conditions. The calculated elastic constants obey the Born-Huang criteria, suggesting that they are mechanically stable at normal and high pressure. Also, the superconducting transition temperature is estimated for the WC and WN in stable structures at atmospheric pressure. [ABSTRACT FROM AUTHOR]

Abstract: First principles calculation were performed using Vienna ab-initio simulation package within the frame work of density functional theory (DFT) to understand the electronic properties of magnesium hydride. At normal pressure, the most stable structure of MgH2 is rutile type with a wide band gap of 3.52eV, which agrees well with the available data. A pressure induced semi-conductor to metallic transition at a pressure of 92.54GPa is predicted. Our results indicate a sequence of pressure induced structural phase transition in MgH2. The obtained sequence of phase transition was α→γ→β→δ→ε at a pressure of 0.37GPa, 3.89GPa,7.23GPa and 11.26GPa, respectively. Thus our results indicate that MgH2 is one of the best hydrogen storage material and the maximum storage capacity achieved was 7.7%. [Copyright &y& Elsevier]