First-principles study of structural,elastic, electronic and optical properties of orthorhombic NaAlF4

We have performed the ab initio total energy calculations using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT) to study the structural parameters, elastic, electronic, chemical bonding and optical properties of orthorhombic NaAlF₄. The calculated lattice parameters are in good agreement with experimental work. The bulk, shear and Young’s modulus, Poisson’s coefficient, compressibility and Lamé’s constants are firstly obtained using Voigt–Reuss–Hill method and the Debye temperature is estimated using Debye-Grüneisen model. Band structure shows a direct band gap at Γ point. Density of states and charge density have been studied, which show the bonding between Na and F is mainly ionic as well as that between Al and F. In order to clarify the mechanism of optical transitions of orthorhombic NaAlF₄, the complex dielectric function, refractive index, extinction coefficient, reflectivity, absorption efficient, loss function and complex conductivity function are calculated. The optical properties and origins of the structure have been analysed.     

Dependence of pressure on elastic,electronic and optical properties of CeO2 and ThO2: A first principles study

The phase transformation of CeO₂ and ThO₂ from fluorite to cotunnite-type structure under pressure is predicted within the density functional theory implemented with the GGA-PW91 method, the pressure induced structural phase transition occurs at 28.9 GPa for CeO₂ and 29.8 GPa for ThO₂. These values are in excellent agreement with the experimentally measured data. The elastic, electronic and optical properties at normal as well as for high-pressure phase have been calculated, particular attention is devoted to the cotunnite phase. Further, the dependence of the elastic constants, the bulk modulus B, the energy band gaps and the dielectric function on the applied pressure are presented.     

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.     

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.     

First-principles study of the structural,elastic and electronic properties of HfTaO3N

HfTaO₃N is an important material in the future industrial applications. However, there are no data in the literature about these analysis. In this paper, structural parameters, elastic, electronic and bonding properties of HfTaO₃N have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). These properties of HfTaO₃N have been firstly theoretically predicted, which await experimental confirmation.     

First-principles study on the structural stabilities,electronic and elastic properties for zirconium under pressure

Using the first-principles calculation based on density-functional theory (DFT), we investigate the pressure-induced phase transitions, electronic and elastic properties of Zr at 0 K. The metal is shown to exhibit a crystal structure sequence of hcp → ω → bcc with increasing pressure. And the transitions happen at 0.14 GPa and 27.01 GPa, respectively. This is in good agreement with the experimental observation. The density of state (DOS) reveals the basic reason for the stability sequence of Zr. The shear moduli c′, c₄₄ and bulk modulus B of bcc Zr all increase with pressure. It is found that bcc Zr satisfies the mechanical stability at pressure beyond 9 GPa. Furthermore, the high-pressure limit of 360 GPa for a stable bcc Zr is deduced for the first time from the cohesive energy calculations. The Mulliken population analysis shows that both s and p electrons transfer to the d orbital with increasing pressure, however, the number of s electrons starts to increase when the pressure exceeds about 100 GPa.     

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.     

First-principles investigations on structural,elastic and electronic properties of SnO2 under pressure

We investigate the structural, elastic, and electronic properties of rutile-type SnO₂ by plane-wave pseudopotential density functional theory method. The lattice constants, bulk modulus and its pressure derivative are all calculated. These properties at equilibrium phase are well consistent with the available experimental and theoretical data. Especially, we study the pressure dependence of elastic properties such as the elastic constants, elastic anisotropy, aggregate acoustic velocities and elastic Debye temperature Θ. It is concluded that this structure becomes more ductile with increasing pressure up to 28 GPa. Moreover, our compressional and shear wave velocities V_P = 7.02 km/s and V_S = 3.84 km/s, as well as elastic Debye temperature Θ = 563 K at 0 GPa compare favorably with the experimental values. The pressure dependences of band structures, energy gap and density of states are also investigated.     

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.     

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.     

FP-LAPW investigation of structural,electronic, linear and nonlinear optical properties of ZnIn2Te4 defect-chalcopyrite

A theoretical study of structural, electronic, linear and nonlinear optical properties of ZnIn₂Te₄ defect-chalcopyrite is presented using the full-potential linearized augmented plane-wave (FP-LAPW) method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA). Moreover, the Engel and Vosko GGA formalism (EV-GGA) is also used to improve the band gap results. The lattice parameters (a, c) and the atomic positions (x, y and z) are optimized and found in good agreements with the available experimental data. Our calculations performed for band structure and density of state show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap of about 0.89 eV for GGA and 1.20 eV for EV-GGA. The linear optical properties namely, the real and imaginary parts of the dielectric function and the reflectivity spectrum are calculated. This compound possesses a considerable negative birefringence. Based on the density functional theory the nonlinear optical properties are calculated and their spectra are analyzed.     

Structural,elastic, electronic and optical properties of new layered semiconductor BaGa2P2

We report the results of a detailed first-principles based density functional theory study of the structural, elastic, electronic and optical properties of a recently synthesized layered semiconductor BaGa₂P₂. The optimized structural parameters are in excellent agreement with the experimental structural findings, which validates the used theoretical method. The single crystal and polycrystalline elastic constants are numerically estimated using the strain–stress method and Voigt–Reuss–Hill approximations. Predicted values of the elastic constants suggest that the considered material is mechanically stable, brittle and very soft material. The three-dimensional surface and its planar projections of Young’s modulus are visualized to illustrate the elastic anisotropy. It is found that Young’s modulus of BaGa₂P₂ show strong dependence on the crystallographic directions. Band structure calculation reveals that BaGa₂P₂ is a direct energy band gap semiconductor. The effective masses of electrons and holes at the minimum of the conduction band and maximum of the valence band are numerically estimated. The density of state, charge density distribution and charge transfers are calculated and analyzed to determine the chemical bonding nature. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron-loss energy function spectra are computed for a wide photon energy range up to 20 eV. Calculated optical spectra exhibit a noticeable anisotropy.     

Facile synthesis,morphological, structural,photocatalytic, and optical properties of ZnFe2O4 nanostructures

Spinel ferrite ZnFe₂O₄ nanostructures have been prepared as sunlight responsive photocatalysts via facile co-precipitation method. The structural, morphological, and optical responses were diligently characterized using XRD, Raman spectroscopy, FESEM, and UV–Vis absorption spectroscopy, respectively. FESEM studies revealed nanoparticles and porous-like nanoparticle aggregates, found to be of cubic spinel ZnFe₂O₄ from XRD and Raman studies. Crystallite size varied from 5 to 13.6 nm, whereas band gap changed from 1.89 to 1.95 eV with CTAB concentration variation. ZnFe₂O₄ nanostructures were employed for sunlight-assisted photodegradation of organic pollutants such as MB, MG, and MO dyes in water. The synthesized ZnFe₂O₄ nanoparticle aggregates with porous-like morphology with crystallite size of 9.2 nm showed superior photocatalytic response and decomposed 80.4% of MB dye in only 40 min. The superiority of the porous-like ZnFe₂O₄ nanoparticle aggregates was mainly ascribed to its optimal crystallite size, narrower band gap, and improved sunlight utilization efficiency. A plausible mechanism of photocatalytic oxidation of dye supported by scavenger studies has also been proposed. The synthesized ZnFe₂O₄ nanostructures have easy magnetic recycling property along with excellent photocatalytic capability and hold potential for the treatment of contaminated water.

     

Structural,elastic, and thermodynamic properties of ZnSexTe1−x: A first-principles study

The structural, elastic, and thermodynamic properties, as well as optical bowing parameters of ZnSe_xTe_1?x ternary alloys are investigated. We adopt the Landau–Lifshitz structural models using the first-principles method, and show that alloys of different structures but identical compositions exhibit highly similar parameters. Results also reveal that the structural relaxation effect plays a crucial role in the gap bowing parameter. The different constituents of the alloys are all mechanically stable. However, they exhibit phase separation at low temperatures because of thermodynamic reasons. The calculated phase diagram shows the temperature at which a stable alloy may be formed for the different compositions of Se. It also demonstrates that the lowest temperature indicating the thermodynamic stability of alloys over the whole composition range (the critical temperature) is about 757 K.     

The electronic and optical properties of Al2O3, MO, and MAl2O4 (M = Zn, Mg)

The electronic properties and the imaginary parts of the dielectric function for nanosized ZnAl2O4 and MgAl2O4 are studied compared with those of B4-ZnO, B1-MgO and alpha-Al2O3 using a first-principles pesudopotential plane-wave method. The results show that both the electronic structures and the optical spectra of ZnAl2O4 and MgAl2O4 are different from those of ZnO, MgO and Al2O3 due to the atomic rearrangement, which agrees with the experimental data. The insight mechanism is also discussed.     

CrSi2能带结构和光学性质的第一性原理研究

采用基于第一性原理的密度泛函理论（DFT）赝势平面波方法，对CrSi2的能带结构、态密度和光学性质进行了理论计算，能带结构计算表明CrSi2属于一种间接带隙半导体，禁带宽度为0．353eV,其能态密度主要由Cr的3d层电子和Si的3p层电子的能态密度决定；计算了CrSi2的介电函数、反射率、折射率及吸收系数等。经比较，计算结果与已有的实验数据符合较好。     

Thickness dependent structural, electronic, and optical properties of Ge nanostructures

In the present paper, we have investigated structural, optical as well as electronic properties of electron beam evaporated Ge thin films having layer thicknesses ranging from ultra-thin (5 nm) to thick (200 nm). The Raman spectra show that all peaks are shifted towards lower wave number as compared to their bulk counterparts and are considered as a signature of nanostructure formation and quantum confinement effect. The Raman line exhibits transformation from nanocrystalline to microcrystalline phase with a reduction in blue shift of peak position with increase in Ge film thickness (>5 nm). Similarly, the optical absorption spectra corresponding to these films also show reduction in blue shift effect, although Ge 5 nm film shows the absorption behaviour quite different from higher thickness films. The corresponding band gap values obtained from absorption measurements are much larger than bulk Ge and are mainly attributed to the effect of quantum confinement as expected for small size particles calculated from GIXRD patterns. AFM data in each case are correlated and discussed with structural as well as optical results to support the effect of growth morphology on the above-mentioned observations. The results are further supported by photoelectron spectroscopy (PES), photoluminescence (PL) and resistivity measurements and are interpreted in terms of crystallinity and quantum confinement effect.     

The energetic and structural properties of bcc NiCu,FeCu alloys: A first-principles study

Using special quasirandom structures (SQS’s), we perform first-principles calculations studying the metastable bcc NiCu and FeCu alloys which occur in Fe–Cu–Ni alloy steels as precipitated second phase. The mixing enthalpies, density of states, and equilibrium lattice parameters of these alloys are reported. The results show that quasi-chemical approach and vegard rule can satisfactorily predict the energetic and structural properties of FeCu alloys but fail to accurately yield that of NiCu. The reason rests with the difference of bond energy variation with composition between NiCu and FeCu alloys induced by competition between ferromagnetic and paramagnetic state. Furthermore, the results show that the energetic and structural properties of these alloys can explain the element distribution of the Cu-rich precipitates in ferrite steels.