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).     

Structural, elastic, electronic, chemical bonding and thermodynamic properties of CaMg2N2 and SrMg2N2: First-principles calculations

We report first-principles density functional theory calculations of the structural, elastic, electronic, chemical bonding and thermodynamic properties of the ternary alkaline earth metal nitrides CaMg₂N₂ and SrMg₂N₂. The calculated equilibrium structural parameters agree well with the experimental findings. Single-crystal and polycrystalline elastic constants and some related properties under pressure effect have been predicted. Both compounds exhibit a striking elastic anisotropy and a ductile behavior. Electronic properties and chemical bonding nature have been studied throughout the band structure, density of states and charge distribution analyses. It is found that these two materials have a direct band gap (Γ-Γ) and a transition to an indirect gap (Γ-M) occurs at about 8.63 and 5.16 GPa in CaMg₂N₂ and SrMg₂N₂, respectively. The chemical bonding has a mixture covalent-ionic character. Thermal effects on some macroscopic properties are predicted using the quasi-harmonic Debye model.     

Ab initio, crystal field and experimental spectroscopic studies of pure and Ni-doped KZnF3 crystals

Pure and Ni²⁺ doped KZnF₃ single crystals were studied using the combination of the DFT-based ab initio methods, crystal field theory and experimental spectroscopic techniques. The electronic, optical and elastic properties have been calculated and compared with available experimental data and good agreement was achieved. Elastic anisotropy of pure KZnF₃ was modeled; calculations of the sound velocity, Debye temperature, Grüneisen parameter and specific heat capacity were performed. Comparison of the calculated results for the pure and doped material, which is reported for the first time for the considered material, enabled to identify the changes in the optical and electronic properties, which are due to the introduced nickel impurity ions. In particular, it was shown that the lowest Ni 3d states appear in the host's band gap at about 1.0 eV above the valence band. The changes of the electron density distribution after doping were also shown. Microscopic analysis of the crystal field effects based on the performed ab initio calculations of the Ni²⁺ density of states at different external pressures enabled to estimate the constants of the electron–vibrational interaction, Huang-Rhys factor, Stokes shift and local bulk modulus around impurity ions. The crystal field calculations of the Ni²⁺ energy levels were performed to analyze and assign the experimental absorption spectrum. Such a combination of the ab initio and semi-empirical calculating techniques leads to a complementary picture of the physical properties of KZnF₃:Ni²⁺ and can be applied to other doped crystals.     

First-principles calculations of structural, electronic and elastic properties of Ca2MgSi2O7

We studied the structural, electronic and elastic properties of åkermanite, Ca₂MgSi₂O₇, by using the first-principles method. The structure of åkermanite is constructed by interleaved tetrahedral and Ca cation layers, and this characteristic is perfectly presented by three-dimensional (3D) crystal lattice, as well as two-dimensional (2D) contour plots of total electron densities in this paper. The chemical bonding and interaction are investigated by analyzing the bond population and density of states (DOS) of the crystal. Theoretical elastic constants of åkermanite are consistent with the experimental values. Moreover, significant anisotropy for Young’s modulus can be observed.     

Structural, electronic and elastic properties of V5Si3 phases from first-principles calculations

The phase stability, electronic, elastic and thermodynamic properties of V₅Si₃ has been investigated by using first-principles calculations based on the density functional theory (DFT). In the present calculations, three phases of V₅Si₃ (Cr₅B₃-prototype, W₅Si₃-prototype and Mn₅Si₃-prototype) have been taken into account to check the phase stability. The calculated formation enthalpies indicate that the W₅Si₃-prototype is the stable phase which is in agreement with experiments, whereas the Cr₅B₃-prototype is a potential metastable phase. The elastic constants, bulk modulus, shear modulus and Young’s modulus are calculated in the present work, and are very close to that of the Nb₅Si₃. The density of states and bonding charge density of V₅Si₃ within the W₅Si₃-phase are obtained indicating a strong covalent character of the bonds. Finally, using the Debye-model, the Debye temperature, heat capacity, and thermal expansion have also been calculated and are in good agreement with experimental results.     

Structural stabilities, electronic and optical properties of CaF2 under high pressure: A first-principles study

The structural stabilities, electronic and optical properties, the pressure-induced metallization for CaF₂ have been studied by using the density functional theory calculations. The ground phase is predicted to transform into Pnma structure at 8.1 GPa, which is well consistent with the experimental findings. Above 278 GPa, Pnma-CaF₂ transform into P6₃/mmc phase. The calculated structural data for and pnma phases are in very good agreement with experimental values. The electronic band structures show that Pnma and P6₃/mmc phases of CaF₂ are insulators at the transition pressure. Upon further compression, the band gap of P6₃/mmc decreases with pressure, and CaF₂ is predicted to undergo metallization around 2250 GPa. The possible reason for the metallization was discussed. All CaF₂ polymorphs have ionic character between Ca–F bond with the analysis of the charge–density distribution and density of states.     

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.     

First-principle calculations of elastic and electronic properties of the filled skutterudite CeFe4P12

A theoretical study of elastic and electronic properties of the filled skutterudite CeFe₄P₁₂ is presented, using the full-potential linear muffin–tin orbital (FP-LMTO) method. In this approach the local spin density approximation (LSDA) was used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus, its pressure derivative and elastic constants. Our calculations performed for band structure and density of state show that this compound is an indirect band gap material (Γ–N). The results are compared with previous calculations and experimental data.     

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 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.     

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.     

FP-APW + lo study of the elastic, electronic and optical properties of the filled skutterudites CeFe4As12 and CeFe4Sb12

Using a full-relativistic version of the full-potential augmented plane wave plus local orbitals (FP-APW + lo) method within the local density approximation (LDA), we have studied the elastic, electronic and optical properties of the filled skutterudites CeFe₄As₁₂ and CeFe₄Sb₁₂. Structural parameters, including lattice constant, internal free parameters and, bulk modulus and its pressure derivative were calculated. We have determined the full set of first-order elastic constants, Young’s modulus, Poisson’s ratio and the Debye temperature of these compounds. Band structures, density of states, pressure coefficients of energy band gaps are also given. It is found that both CeFe₄As₁₂ and CeFe₄Sb₁₂ are indirect band gap semiconductors. The valence band maximum (VBM) is located at Γ point, whereas the conduction band minimum (CBM) is located at N point. Optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss were calculated for radiation up to 30 eV. This is the first quantitative theoretical prediction of the elastic and optical properties for these compounds, and it still awaits experimental confirmation.     

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.     

Ab initio study of antisite defective layered Ge2Sb2Te5

By means of ab initio calculations, we have investigated the antisite defects in layered Ge₂Sb₂Te₅ (GST). Our results show that both Te_Sb and Sb_Te antisite defective GST alloys are energetically favorable and mechanically stable. Furthermore, the presence of antisite defects results in the decrease in band gaps and hence the increase in the electrical conductivity, while shows slight effect on chemical bonding characters. Based on the present results, increased electrical conductivity and decreased thermal conductivity are expected by introducing antisite defects in GST related layered materials.     

Ab initio study of the structural, elastic, electronic and optical properties of the antiperovskite SbNMg3

A theoretical study of structural, elastic, electronic and optical properties of the cubic antiperovskite SbNMg₃ is presented using the pseudo-potential plane wave method (PP-PW) within the generalized gradient approximation (GGA). Results are given for lattice constant, elastic constants and their pressure dependence. Band structure, density of states and pressure coefficients of energy gaps are also given. Furthermore, the optical reflectivity, refractive index, extinction coefficient, dielectric function and electron energy loss are calculated for radiation up to 30 eV. The results are compared with the available theoretical and experimental data.     

Prediction study of the structural, elastic and electronic properties of ANSr3 (A = As, Sb and Bi)

Based on first-principles total energy calculations, we predict the elastic and electronic properties of the anti-perovskites AsNSr₃, SbNSr₃ and BiNSr₃ compounds. The calculated lattice constants are in good agreement with the available results. The independent elastic constants (C₁₁, C₁₂ and C₄₄) and their pressure dependence are calculated using the static finite strain technique. The isotropic elastic moduli, namely, bulk modulus (B), shear modulus (G), Young’s modulus (E), Poisson’s ratio (σ) and Lame’s constants (λ and μ) are calculated in framework of the Voigt–Reuss–Hill approximation for ideal polycrystalline ANSr₃ aggregates. By analysing the ratio between the bulk and shear moduli, we conclude that ANSr₃ compounds are brittle in nature. We estimated the Debye temperature of ANSr₃ from the average sound velocity. The band structures show that all studied materials are semiconductors. The analysis of the site and momentum projected densities, charge transfer and charge densities show that bonding is of covalent–ionic nature. This is the first quantitative theoretical prediction of the elastic and electronic properties of AsNSr₃, SbNSr₃ and BiNSr₃ compounds that requires experimental confirmation.     

First-principles studies of the electronic and elastic properties of metal nitrides XN (X = Sc, Ti, V, Cr, Zr, Nb)

Electronic and elastic properties of a series of the transition metal ion mononitrides (ScN, TiN, VN, CrN, ZrN, NbN) have been modeled in the framework of ab initio plane wave spin-polarized calculations using the generalized gradient and local density approximations. The calculated band structures are typical for metallic compounds, except for ScN, whose band structure is that one of the gapless semiconductor. Strongly delocalized d states of transition metal ions are spread over a wide region of about 10-12 eV and are strongly hybridized with the nitrogen 2p states. Among the considered nitrides, only CrN exhibits a clear difference between the spin-up and spin-down states, which would manifest itself in magnetic properties. The overall appearance of the calculated cross-sections of the electron density difference changes drastically when going from Sc to Nb in the considered series of compounds. For the first time the calculated tensors of the elastic constants and elastic compliance constants were used for the analysis and visualization of the directional dependence of the Young’s moduli. It was shown that ScN and VN can be characterized as more or less elastically isotropic materials, whereas in TiN, CrN, ZrN, and NbN the Young’s moduli vary significantly in different directions. The maximal values of the Young’s moduli are along the crystallographic axes, the minimal values are along the bisector direction in the coordinate planes; the difference between them in the case of CrN exceeds one order of magnitude. In addition, pressure dependence of the “metal - nitrogen” distance was modeled.     

Structure and electronic properties of transition metal dichalcogenide MX2 (M = Mo,W, Nb; X = S,Se) monolayers with grain boundaries

Layered transition metal dichalcogenides with unique mechanical, electronic, optical, and chemical properties can be used for novel nanoelectronic and optoelectronic devices. Large-area monolayers synthesized using chemical vapor deposition are often polycrystals with many dislocations and grain boundaries (GBs). In the present paper, atomic structure and electronic properties of MX₂ (M = Mo, W, Nb; X = S, Se) with the GBs were investigated using first principles based on density functional theory. Simulation results revealed that the zigzag-oriented GBs (which consist of pentagon/heptagons (5-7) pairs) were more stable than the armchair-oriented GBs (which consist of pentagon/heptagons (5-7-5-7) pairs). The GBs induced defect levels are located within the band gap for the semiconductor materials of MX₂ (M = Mo, W; X = S, Se) monolayers, and the NbS₂ and NbSe₂ remained as metallic materials with GBs. Results provided a possible pathway to build these nano-layered materials into nanoelectronic devices.     

Effects of spin-orbit coupling on various properties of hafnium dihydride

First-principles calculation reveals that the spin-orbit coupling (SO) should have an important effect on lowering the total energy of each HfH_x (x = 1, 1.25, 1.5, 1.75, and 2) phase with the FCC (δ) and FCT (γ and ?) structures, while has a negligible effect on structural stability of various HfH_x phases as well as atomic structure and lattice constants of FCC HfH_x phases. Calculations also show that mechanical stability plays a more significant role than thermodynamics in determining the existence of various HfH_x phases as well as phase transitions between δ, γ, and ?. Moreover, the intrinsic composition range of the δ → ? transition from the present study is 1.75 ≤ x ≤ 2, which could clarify the controversy of experimental observations in the literature. In addition, the splitting of degenerate bands and the change of density of states at Fermi level would bring about a deep understanding of the effect of SO on stability of various HfH_x phases.