First-principles study of structural stabilities,elastic and electronic properties of transition metal monocarbides (TMCs) and mononitrides (TMNs)

The structural stabilities, elastic and electronic properties of 5d transition metal mononitrides (TMNs) XN with (X = Ir, Os, Re, W and Ta) and 5d transition metal monocarbides (TMCs) XC with (X = Ir, Os, Re and Ta) were investigated using the full-potential linear muffin-tin orbital (FP-LMTO) method, in the framework of the density functional theory (DFT) within the local density approximation (LDA) for the exchange correlation functional. The ground state quantities such as the lattice parameter, bulks modulus and its pressure derivatives for the six considered crystal structures, Rock-salt (B1), CsCl (B2), zinc-blend (B3), Wurtzite (B4), NiAs (B8₁) and the tungsten carbides (B_h) are calculated. The elastic constants of TMNs and TMCs compounds in its different stable phases are determined by using the total energy variation with strain technique. The elastic modulus for polycrystalline materials, shear modulus (G), Young's modulus (E), and Poisson's ratio (ν) are calculated. The Debye temperature (θ_D) and sound velocities (v_m) were also derived from the obtained elastic modulus. The analysis of the hardness of the herein studied compounds classifies OsN – (B4 et B8₁), ReN – (B8₁), WN – (B8₁) and OsC – (B8₁) as superhard materials. Our results for the band structure and densities of states (DOS), show that TMNs and TMCs compounds in theirs energetically and mechanically stable phase has metallic characteristic with strong covalent nature Metal–Nonmetal elements.     

First principles study on the elastic properties and electronic structures of (Fe, Cr)3C

First principles calculations are performed to investigate the elastic properties and electronic structures of Cr doped Fe₃C carbides, the obtained results are compared with Cr₃C and Fe₃C. The calculated bulk modulus of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ is 260GPa and 270GPa, respectively, larger than Fe₃C. So the hardness of Fe₃C phase can be enhanced by doped with an appropriate amount of Cr, however, the calculated formation enthalpy and defect formation enthalpy of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are positive. On the other hand, the electronic calculations reveal that the ground states of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are ferromagnetic. The evaluated local magnetic moments of Fe at 4c sites are larger than that of 8d sites, which is analogous to Fe₃C. Milliken population results indicate that the stabilities of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are reduced mainly due to the strong repulsive bonds among metal atoms.     

Effect of Y and Zn substitution on elastic properties of 6H-type ABCBCB LPSO structure in Mg97Zn1Y2 alloy

Theoretical investigations of the effect of Y and Zn atom substitution on elastic properties of 6H-type ABCBCB LPSO structure in Mg₉₇Zn₁Y₂ alloy have been performed from density function theory. Elastic properties, including elastic constants and elastic modulus were investigated, and the influence of Y and Zn substitution were discussed in detail. Elastic anisotropies were analyzed by several methods, and the results show that the anisotropy in compression is almost negligible, whereas the anisotropy in shear is relatively large. Furthermore, the shear anisotropy becomes larger with Zn substitution than Y substitution. The electronic characteristics indicate that the Mg-Y and Mg-Zn bonds exhibit covalent feature due to hybridization, so the interactions of Mg with Y and Zn are enhanced.     

Effects of alloying elements on elastic properties of Ni by first-principles calculations

The effects of alloying elements (Al, Co, Cr, Cu, Fe, Hf, Mo, Nb, Pt, Re, Ta, Ti, W, Y and Zr) on the elastic constants (c_ij’s) of Ni have been investigated using first-principles calculations within the generalized gradient approximation. The results are compared with the available experimental data and analyzed based on the volume changes and electron density. It is found that the shear modulus decreases with increasing volume caused by alloying addition and the bulk modulus (B) is related to the total molar volume (V_m) and electron density (n) with the relationship, . The melting temperatures of Ni–X dilute solutions calculated from the available thermodynamic databases have been compared to those obtained from the empirical relationship with the elastic constant c₁₁. The calculated elastic constants show good relationships with the mechanical properties such as 0.2% flow stress and give us a guideline to understand and develop Ni-based superalloys.     

The electronic, elastic and structural properties of Pd-Zr intermetallic

A first-principles plane-wave pseudopotential method based on the density functional theory was used to investigate the energetic, electronic structures and elastic properties of intermetallic compounds of Pd-Zr system. The Enthalpies of formation, the cohesive energies and elastic constants of these compounds were estimated from the electronic structure calculations and their structural stability was also analyzed. The results show that the PdZr₂ compound is stable, relative to other compounds, and as the concentration of Pd increases, the enthalpy of formation gradually increased except Pd₄Zr_3.The calculated elastic constants are then used to estimate mechanical properties of Pd-Zr intermetallics compounds. The brittle/ductile behavior is assessed by analyzing the phenomenological formula G/B of shear modulus (G) over bulk modulus (B). The new knowledge from this study could be used for future development of Pd-Zr system.     

First-principles calculations of structural, electronic, and elastic properties of MgZrSi2O7

For the first time, the recently synthesized pyrochlore MgZrSi₂O₇ [J. Xu et al., Mater. Chem. Phys. 128 (2011) 410] has been analyzed using the first principles calculations. The electronic and elastic properties were predicted; in particular, the band gap is indirect and has the value of 6.75 eV. The bulk modulus equals to 186.51 ± 1.95 GPa. Anisotropy of elastic properties was analyzed by comparing the upper and lower estimates of the shear moduli. In addition, directional dependence of the Young's modulus was calculated and visualized; its value varies in the range from 249.7 GPa (along the a, b, c crystallographic axes) to 136.84 GPa (along the bisector direction in any of the ab, bc, ac planes).     

First-principles calculations of the structural,electronic and elastic properties of ZnWO4 and CdWO4 single crystals at the ambient and elevated pressure

Two technologically important tungstate crystals – CdWO₄ and ZnWO₄ – are studied theoretically using the plane wave based first-principles calculations. The optimized crystal structures were used to calculate the structural, electronic and elastic properties of both materials under varying hydrostatic pressure. It was shown that the band gaps, which are direct at ambient pressure, turn into indirect ones in both compounds in the pressure range from 5 to 10 GPa, whereas the values of the band gaps themselves depend only slightly on the applied pressure. Differences in compressibility along the crystallographic axes and the W–O, Cd–O, Zn–O chemical bonds were revealed and quantified by calculating the pressure coefficients for all these characteristic distances. The first estimations of the complete elastic tensor constants for both materials are reported.     

Elastic and mechanical properties of intrinsic and doped PbSe and PbTe studied by first-principles

Lead chalcogenides, most notably lead selenide (PbSe) and lead telluride (PbTe), have become an active area of research due to their thermoelectric (TE) properties. The high figure of merit of these materials has brought much attention to them, due to their ability to convert waste heat into electricity. Recent efforts, such as applying pressure or doping, have shown an increase in TE efficiency. Variation in application and synthesis conditions gives rise to a need for analysis of mechanical properties of these materials. In addition to the rocksalt (NaCl) structure at ambient conditions, lead chalcogenides have an orthorhombic (Pnma) intermediate pressure phase and a higher pressure CsCl phase. By using first-principles calculations, performed within density functional theory, we study the structural, elastic and mechanical properties of PbTe and PbSe in their three phases. For each phase, elastic constants, bulk modulus, shear modulus, and Young's modulus are calculated, and the NaCl phase is studied with typical dopants, both n-type (Bi and I) and p-type (Na, In, and Tl). Pugh's ratio is employed to give insight on the brittleness of the materials and phase studied. The results presented here will be useful to guide future experiments toward the search for structurally stable TE materials.     

Electronic and optical properties of the orthorhombic compounds FeX2 (X = P,As and Sb)

First principles calculations, by means of the full-potential linearized augmented plane wave (FP-LAPW) method within the local density approximation (LDA), were carried out for the structural, electronic and optical properties of the orthorhombic compounds FeP₂, FeAs₂ and FeSb₂. The structural properties are determined through the total energy minimization and the relaxation of the internal parameters. The modified Becke–Johnson (mBJ) method is applied for the electronic structure of FeSb₂. Our LDA-calculation shows that the first two compounds are indirect-gap semiconductors, while for the third one it predicts a small hole-pocket at the R point. The mBJ gives a semiconducting state with an indirect energy gap of 0.248 eV for FeSb₂. The overall shape of the calculated imaginary parts of the dielectric tensor is similar for the three compounds. The assignment of the structures in the optical spectra and band structure transitions are investigated. The electronic dielectric constant along (0 1 0) direction is the largest for the three compounds. For FeAs₂, the calculated components of reflectivity have the same trend of variation as the measured ones in the energy range 1.54–3.1 eV.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

Thermal stability and elastic properties of Mg2X (X = Si, Ge, Sn, Pb) phases from first-principle calculations

Thermal stabilities, elastic properties and electronic structures of Mg₂Si, Mg₂Ge, Mg₂Sn and Mg₂Pb have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Mg₂Ge has the strongest alloying ability and Mg₂Si has the highest structural stability. Gibbs free energy, heat capacity and Debye temperature are calculated and discussed. The elastic parameters are calculated, the bulk moduli, shear moduli, Young’s moduli and poisson ratio value are derived, the brittleness and plasticity of these phases are discussed, and the brittle behavior and structural stability mechanism are also explained through the densities of states (DOS) of these intermetallic compounds.     

Theoretical studies of the pressure-induced zinc-blende to cinnabar phase transition in CdTe and thermodynamical properties of each phase

Luminescence of CdTe quantum dots embedded in ZnTe is quenched at pressure of about 4.5 GPa in the high-pressure experiments. This pressure-induced quenching is attributed to the “zinc-blende–cinnabar” phase transition in CdTe, which was confirmed by the first-principles calculations. Theoretical analysis of the pressure at which the phase transition occurs for CdTe was performed using the CASTEP module of Materials Studio package with both generalized gradient approximation (GGA) and local density approximation (LDA). The calculated phase transition pressures are equal to about 4.4 GPa and 2.6 GPa, according to the GGA and LDA calculations, respectively, which is in a good agreement with the experimental value. Theoretically estimated value of the pressure coefficient of the band-gap luminescence in zinc-blende structure is in very good agreement with that recently measured in the QDs structures. The calculated Debye temperature, elastic constants and specific heat capacity for the zinc-blend structure agree well with the experimental data; the data for the cinnabar phase are reported here for the first time to the best of the authors' knowledge.     

First principles study of structural,electronic and elastic properties of lutatium mono-pnictides

The structural, electronic, elastic and thermal properties of two lutatium mono-pnictides (LuAs and LuSb) have been studied using the density functional theory within the generalized gradient approximation. The calculations indicate that there is a structural phase transition from their ambient NaCl – (B₁) to CsCl – (B₂) structure at 56.7 and 25.2 GPa along with the volume collapse percentage of 3% and 5%, respectively. Structural parameters like lattice constant (a₀), bulk modulus (B) and pressure derivative of the bulk modulus (B′) are presented. The calculated band structures indicate that B₁ and B₂ phase of these compounds are metallic. We have calculated the second order elastic constants for these compounds. We also compare the ground state (a₀ and B) and high pressure phase transition (P_t) properties for three members of lanthanide series.     

Stable structure of platinum carbides: A first principles investigation on the structure,elastic, electronic and phonon properties

A comprehensive first principles study of structural, elastic, electronic, phonon and thermodynamical properties of novel metal carbide, platinum carbide (PtC) is reported within the density functional theory scheme. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus and finally the enthalpy of PtC in zinc blende (ZB) and rock-salt (RS) structures are determined. The energy band structure and electron density of states for the two phases of PtC are also presented. Of these phases zinc blende phase of PtC is found stable and phase transition from ZB to RS structure occurs at the pressure of about 37.58 GPa. The phonon dispersion curves and phonon DOS are also presented. All positive phonon modes in phonon dispersion curves of ZB-PtC phase indicate a stable phase for this structure. Within the GGA and harmonic approximation, thermodynamical properties are also investigated. All results reveal that the synthesized PtC would favor ZB phase. The compound is stiffer and ductile in nature.     

Structural,thermodynamic, electronic,and optical properties of NaH from first-principles calculations

The structural stability, thermodynamic, electronic, and optical properties of NaH with rock salt (B1) structure and cesium chloride (B2) structure under high pressure are investigated by first-principles calculations using norm-conserving pseudopotential applying a generalized gradient approximation (GGA) for exchange and correlation. Through the analysis of energy–volume variation, we find the phase transition of NaH from B1 to B2 structure occurs at 32.3 GPa, which in good agreement with the diamond-anvil-cell high-pressure experimental value of 29.3 ± 0.9 GPa [Phys. Rev. B 36 (1987) 7664]. By using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature Θ_D, heat capacity C_V, thermal expansion coefficient α, and Grüneisen parameter γ are successfully obtained in the temperature range from 0 to 700 K and pressure ranges from 0 to 32 GPa and 33 to 100 GPa for NaH B1 and B2 phases, respectively. Analysis of band structures suggests that the NaH has an indirect band gap that the valence band maximum is at the W point and the conduction minimum locates at L point. The calculated energy gaps is very close to that value obtained in recent full potential augmented plane wave calculations. The optical properties including dielectric function ?(ω), absorption coefficient α(ω), reflectivity coefficient R(ω), and refractive index n(ω) are also calculated and analyzed.     

Theoretical prediction of the structural, electronic and optical properties of SnB2O4 (B = Mg, Zn, Cd)

The structural, electronic and optical properties of the cubic spinels SnB₂O₄, with B = Mg, Zn and Cd, were studied by means of the full-potential (linear) augmented plane wave plus local orbitals method within the local density and generalized gradient approximations for the exchange-correlation potential. The Engel-Vosko form of the generalized gradient approximation (EV-GGA), which better optimizes the potential for the band structures, was also used. The results of bulk properties, including lattice constants, internal parameters, bulk moduli and their pressure derivatives are in good agreement with the literature data. The band structures show a direct band gap (Γ-Γ) for the three compounds. The computed band gaps using the EV-GGA show a significant improvement over the more common GGA. All the calculated band gaps increase with increasing pressure and fit well to a quadratic function. Analysis of the density of states revealed that the lowering of the direct gap (Γ-Γ) from SnMg₂O₄ to SnZn₂O₄ to SnCd₂O₄ can be attributed to the p-d mixing in the upper valence band of SnZn₂O₄ and SnCd₂O₄. We present calculations of the frequency-dependent complex dielectric function ?(ω). We find that the values of zero-frequency limit ?₁(0) increase with decreasing the energy band gap. The origin of the peaks and structures in the optical spectra is determined in terms of the calculated energy band structures.     

Effects of grafted carboxyl groups on structural stability and elastic properties of graphene

Molecular dynamics is employed to study the effect of grafted carboxyl groups on elasticity, high-temperature structural stability and compressive properties of armchair and zigzag graphene nanoribbons (AGNRs and ZGNRs). It is found that the Young's moduli decrease as the graft quantity increases from no graft to full grafts (–COOH). The fall for AGNR is larger than that for ZGNRs, which further increases the anisotropy of the elastic property. The structures after undergoing high temperature at 1500 K show that the AGNR exhibits less distortion than ZGNR and grafting carboxyl can obviously reduce structural deformation. Optimizing the GNRs with different in-plane compressive strains, it is evident that the buckling strain (∼11.1%) of ZGNR is larger than that (∼8.1%) of AGNRs, but the AGNR–COOH does not exhibit buckling until the strain is 9.7%, while ZGNR–COOH exhibits buckling at strain of 7.9%. The results indicate that grafting carboxyl reduces the elastic moduli of GNRs, but significantly improves structural stability at high temperature. Grafting carboxyl enhances resistance to compressive buckling of AGNR, but is bad for ZGNR.     

Revealing the role of site occupation in phase stability,magnetic and electronic properties of Ni-Mn-In alloys by ab initio approach

The effects of site occupation on the phase stability,martensitic transformation,and the magnetic and electronic properties of a full series of Ni-Mn-In alloys are theoretically studied by using the ab initio calculations.Results indicate that the excess atoms of the rich component directly take the sublattices of the deficient components of the Ni2Mn1+xIn1-x,Ni2-xMn1+xIn,and Ni2+xMn1-xIn alloys.Nevertheless,the mixed and indirect site occupations may coexist in the Ni2+xMnIn1-x system.The relevant magnetic configurations of the austenite for the four alloy systems have also been determined.The results show that,except for the austenite in the Ni2-xMn1+xIn alloys,which tend to be ferrimagnetic,the other alloys all present ferromagnetic austenite.Thus,the site occupation and associated magnetic states are the crucial influencing factors of the phase stability,martensitic transformation,and the total magnetic moment.The electronic structure of the austenite phase also shows that the covalent bonding plays an important role in the phase stability.The key finding of this work is both Ni2Mn1+xIn1-x and Ni2+xMnIn1-x alloys serve as the potential shape memory alloys.     

Elastic properties and sound velocities of silicane/graphane hybrids

Using ab initio density functional theory, the effect of hydrogen arrangement on the elastic properties of silicene–graphene hybrid is studied. Mechanical stability, elastic constants and sound velocities of pure and five configurations of hydrogenated SiC sheet, namely, chair, table, boat, zigzag and armchair, are explicitly examined. To reveal the anisotropic properties of the six structures, the polar plots of Young’s modulus, Poisson ratio and acoustic waves speed are given. Compared to graphene, it is shown that all the isotropic systems are less stiffer with lower in-plane Young’s modulus and stronger with their larger Poisson ratio, moreover, their compressional and shear waves propagate faster. The analysis of linear elastic behavior shows that the armchair configuration has an auxetic structure. The result of this work could be used for the design of future silicane–graphane based nanodevices with potentially large technological impact in nanomechanics.