Theoretical investigation on the electronic structure, elastic properties, and intrinsic hardness of Si2N2O

According to the density functional theory we systematically study the electronic structure, the mechanical prop- erties and the intrinsic hardness of Si2N2O polymorphs using the first-principles method. The elastic constants of four Si2N2O structures are obtained using the stress-strain method. The mechanical moduli (bulk modulus, Young’s mod- ulus, and shear modulus) are evaluated using the Voigt-Reuss-Hill approach. It is found that the tetragonal Si2N2O exhibits a larger mechanical modulus than the other phases. Some empirical methods are used to calculate the Vickers hardnesses of the Si2N2O structures. We further estimate the Vickers hardnesses of the four Si2N2O crystal structures, suggesting all Si2N2O phases are not the superhard compounds. The results imply that the tetragonal Si2N2O is the hardest phase. The hardness of tetragonal Si2N2O is 31.52 GPa which is close to values of β-Si3N4 and γ-Si3N4.     

Elastic property and intrinsic hardness of novel superhard ternary nitrides (CSi2N4 and SiC2N4)

In this work, the new ternary nitrides (CSi₂N₄ and SiC₂N₄) are designed by the substitution method. The structures, elastic properties, intrinsic hardness and Debye temperature of the new ternary nitrides (CSi₂N₄ and SiC₂N₄) are studied by first-principles calculations based on the density-functional theory. The elastic constants C_ij$C_{ij}$ of these new ternary nitrides are obtained using the stress–strain method. Derived elastic constants, such as bulk modulus, shear modulus, Young's modulus, Poisson coefficient and brittle/ductile behavior are estimated using Voigt–Reuss–Hill theories. The results indicate that γ-CSi₂N₄, p-CSi₂N₄ and p-SiC₂N₄ are mechanically stable. Calculated B/G$B/G$ values and Poisson's ratio for γ-CSi₂N₄, p-CSi₂N₄ and p-SiC₂N4 indicate that these materials are brittle. The calculated anisotropy parameters indicate that γ-CSi₂N₄ shows weak anisotropy and p-SiC₂N₄ and p-CSi₂N₄ have larger anisotropy. Based on the microscopic hardness model, p-CSi₂N₄, p-SiC₂N₄ and γ-CSi₂N₄ should be viewed as superhard materials with some peculiar mechanical properties.     

Theoretical investigation on the electronic structure,elastic properties, and intrinsic hardness of Si2N20

According to the density functional theory we systematically study the electronic structure, the mechanical prop- erties and the intrinsic hardness of Si2N2O polymorphs using the first-principles method. The elastic constants of four Si2N2O structures are obtained using the stress-strain method. The mechanical moduli （bulk modulus, Young’s mod-ulus, and shear modulus） are evaluated using the Voigt-Reuss-Hill approach. It is found that the tetragonal Si2N2O exhibits a larger mechanical modulus than the other phases. Some empirical methods are used to calculate the Vickers hardnesses of the Si2N2O structures. We further estimate the Vickers hardnesses of the four Si2N2O crystal structures, suggesting all Si2N2O phases are not the superhard compounds. The results imply that the tetragonal Si2N2O is the hardest phase. The hardness of tetragonal Si2N2O is 31.52 GPa which is close to values of β-Si3N4 and γ-Si3N4.     

The structural, electronic, elastic and optical properties of AlCu(Se1−xTex)2 compounds from first-principle calculations

The structural, electronic structure, elastic and optical properties of the AlCu(Se_1−xTe_x)₂ compounds have been investigated by using a first-principles method based on density functional theory. The lattice constants of the quaternary compounds AlCu(Se_1−xTe_x)₂ increase with the increasing of Te composition. The calculated lattice constants for the ternary compounds i.e. AlCuSe₂ and AlCuTe₂ are in good agreement with the experimental data. The band structures show that the compounds have direct band gap and the band gaps are found to vary nonlinearly with composition. The total and part density of states of the quaternary AlCu(Se_1−xTe_x)₂ compounds are discussed. The calculated elastic constants indicate that all of the AlCu(Se_1−xTe_x)₂ compounds are mechanically stable. The bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν can be obtained by using the Voigt-Reuss-Hill averaging scheme. The B/G ratios of the AlCu(Se_1−xTe_x)₂ compounds indicate that AlCu(Se_0.8Te_0.2)₂ is ductile and the others are brittle. The Debye temperature of the AlCu(Se_1−xTe_x)₂ compounds decreases a little with increasing Te content except the compound with x = 0.4. The dielectric functions, refractive index, extinction coefficient, absorption spectrums and energy-loss function of the AlCuSe₂ and AlCuTe₂ are also calculated and discussed in this work.     

Structural, electronic, elastic and optical properties of fluoro-perovskite KZnF3

We determine the structural, electronic, elastic and optical properties of fluoro-perovskite KZnF₃ using the full potential linear augmented plane wave approach (FP-LAPW) based on the density functional theory (DFT). The exchange-correlation potential is treated by the local density approximation (LDA) and the generalized gradient approximation (GGA). The calculated structural parameters are in good agreement with the available data. We have obtained an indirect band gap. The effect of the pressure on the band gaps is investigated. We evaluate the elastic constants (C_ij), elastic moduli and the Debye temperature. The imaginary and the real parts of the dielectric function ε(ω) and some optical constants are also calculated.     

Structural,electronic and optical properties of Ca3Bi2: First-principles investigation

In this work by applying first principles calculations structural, electronic and optical properties of Ca₃Bi₂ compound in hexagonal and cubic phases are studied within the framework of the density functional theory using the full potential linearized augmented plane wave (FP-LAPW) approach. According to our study band gap for Ca₃Bi₂ in hexagonal phase are 0.47, 0.96 and 1?eV within the PBE-GGA, EV-GGA and mBJ-GGA, respectively. The corresponding values for cubic phase are 1.24, 2.08 and 2.14?eV, respectively. The effects of hydrostatic pressure on the behavior of the electronic properties such as band gap, valence bandwidths and anti-symmetry gap are investigated. It is found that the hydrostatic pressure increases the band widths of all bands below the Fermi energy while it decreases the band gap and the anti-symmetry gap. In our calculations, the dielectric tensor is derived within the random phase approximation (RPA). The first absorption peak in imaginary part of dielectric function for both phases is located in the energy range 2.0–2.5?eV which are beneficial to practical applications in optoelectronic devices in the visible spectral range. For instance, hexagonal phase of Ca₃Bi₂ with a band gap around 1?eV can be applied for photovoltaic application and cubic phase with a band gap of 2?eV can be used for water splitting application. Moreover, we found the optical spectra of hexagonal phase are anisotropic along E||x and E||z.     

The structural, elastic, and electronic properties of the pyrite-type phase for SnO2

We have studied some structural, thermodynamic, elastic, and electronic properties of pyrite-type SnO₂ polymorph by performing ab initio calculations within the LDA approximation. The basic physical properties, in particular lattice constant, bulk modulus, second-order elastic constants (C_ij), and the electronic structure, are calculated, and compared with the available experimental data. In order to gain some further information on the mechanical properties, we have also calculated the Young's modulus, Poison's ratio (ν), anisotropy factor (A), sound velocities, and Debye temperature for the same compound.     

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

The structural, electronic, elastic and optical properties as well as phase transition under pressure of SrTe have been systematically investigated by first-principles pesudopotential calculations. Five possible phases of SrTe have been considered. Our results show that SrTe undergoes a phase transition from NaCl-type (B1) to CsCl-type (B2) structure at 10.9 GPa with a volume collapse of 9.43%, and no further transition is found. We find that SrTe prefer h-MgO instead of wurtzite (B4) structure for its metastable phase because that the ionic compound prefers a high coordination. The elastic moduli, energy band structures, real and imaginary parts of the dielectric functions have been calculated for all considered phases, and we find that a smaller energy gap yields a larger high-frequency dielectric constant. Our calculated results are discussed and compared with the available experimental and theoretical data.     

Theoretical investigation on the electronic structure,elastic properties,and intrinsic hardness of Si₂N₂O

According to the density functional theory we systematically study the electronic structure, the mechanical prop- erties and the intrinsic hardness of Si2N2O polymorphs using the first-principles method. The elastic constants of four Si2N2O structures are obtained using the stress-strain method. The mechanical moduli (bulk modulus, Young’s mod- ulus, and shear modulus) are evaluated using the Voigt-Reuss-Hill approach. It is found that the tetragonal Si2N2O exhibits a larger mechanical modulus than the other phases. Some empirical methods are used to calculate the Vickers hardnesses of the Si2N2O structures. We further estimate the Vickers hardnesses of the four Si2N2O crystal structures, suggesting all Si2N2O phases are not the superhard compounds. The results imply that the tetragonal Si2N2O is the hardest phase. The hardness of tetragonal Si2N2O is 31.52 GPa which is close to values of β-Si3N4 and γ-Si3N4.     

Structural, elastic, electronic and optical properties of a new layered-ternary Ta4SiC3 compound

We propose a new layered-ternary Ta₄SiC₃ with two different stacking sequences (α- and β-phases) of the metal atoms along c axis and study their structural stability. The mechanical, electronic and optical properties are then calculated and compared with those of other compounds M₄AX₃ (M=V, Nb, Ta; A=Al, Si and X=C). The predicted compound in the α-phase is found to possess higher bulk modulus than these compounds. The independent elastic constants of the two phases are also evaluated and the results discussed. The electronic band structures for α- and β-Ta₄SiC₃ show metallic conductivity. Ta 5d electrons are mainly contributing to the total density of states (DOS). We see that the hybridization peak of Ta 5d and C 2p lies lower in energy and the Ta 5d-C 2p bond is stronger than Ta 5d-Si 3p bond. Further an analysis of the different optical properties shows the compound to possess improved behavior compared to similar types of compounds.     

Structural stability and elastic properties of L12 Co3(Ga,W) precipitate from first-principle calculations

Ultrasoft pseudopotential within a generalized gradient approximation was employed to study the structural stability, electronic structure, and elastic properties of ternary Co₃(Ga,W) precipitate. The Young’s and shear moduli of the polycrystals containing the Co₃(Ga,W) precipitate were calculated using the Voigt-Reuss-Hill averaging scheme. Results show that the stable ternary Co₃(Ga,W) compound has the L1₂ structure, and is ductile in nature. The structural stability of the Co₃(Ga,W) compound is discussed together with the calculated electronic structure.     

Structural and optical properties of WO3-ZnO-PbO-B2O3 glasses

Glass samples of compositions 20PbO-80B₂O₃ and xWO₃—(20−x) ZnO-20PbO-60B₂O₃ with x varying from 0% to 10% mole fraction are prepared by the melt quench technique. Decrease in the band gap from 2.86 to 2.16 eV for ZnO-PbO-B₂O₃ glasses with an increase in the WO₃ content has been observed and discussed. The FTIR spectral studies have pointed out the conversion of structural units of BO₃ to BO₄ and WO₄ to WO₆ with the presence of W-O-W vibration of tungsten and incorporation of ZnO₄ structural units of zinc in these glasses. The increase in density from 2.75 to 4.03 gcm⁻³ for ZnO-PbO-B₂O₃ glasses is observed with an increase in WO₃ content. Due to the formation of WO₆, WO₄ and BO₄ units, changes in the atomic structure with WO₃ composition are observed and discussed.     

Pressure dependence of dielectric constant,elastic constants,and lattice parameters of the Y3(Ga,Al)5O12 host

Atomistic simulations were performed to investigate the lattice parameters, dielectric constant, and elastic constants of Y₃(Ga_xAl_5−x)O₁₂ (x = 1, 2, 3, 4, 5) structures. The calculated lattice parameters and elastic constants are in good agreement with those in available experimental results. The pressure dependence of all studied quantities was investigated. In general, a change in the behavior of all studied quantities is found when the Ga concentration becomes more than that of the aluminum (Al) in Y₃(Ga_xAl_5−x)O₁₂ (x = 1, 2, 3, 4, 5) structures.     

Structural,elastic and thermodynamic properties under pressure and temperature effects of MgIn2S4 and CdIn2S4

A density functional-based method is used to investigate the structural, elastic and thermodynamic properties of the cubic spinel semiconductors MgIn₂S₄ and CdIn₂S₄ at different pressures and temperatures. Computed ground structural parameters are in good agreement with the available experimental data. Single-crystal elastic parameters are calculated for pressure up to 10 GPa and temperature up to 1200 K. The obtained elastic constants values satisfy the requirement of mechanical stability, indicating that MgIn₂S₄ and CdIn₂S₄ compounds could be stable in the investigated pressure range. Isotropic elastic parameters for ideal polycrystalline MgIn₂S₄ and CdIn₂S₄ aggregates are computed in the framework of the Voigt–Reuss–Hill approximation. Pressure and thermal effects on some macroscopic properties such as lattice constant, volume expansion coefficient and heat capacities are predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account.     

Structural and optical properties of thermally evaporated Bi2Te3 films

Bi₂Te₃ films were prepared by thermal evaporation technique. X-ray diffraction analysis for as-deposited and annealed films in vacuum at 150 °C were polycrystalline with rhombohedral structure. The crystallite size is found to increase as the film thickness increases and has values in the range 67–162 nm. The optical constants (the refractive index, n, and absorption index, k) were determined using transmittance and reflectance data in the spectral range 2.5–10 μm for Bi₂Te₃ films with different thicknesses (25–99.5 nm). Both n and k are independent on the film thickness in the investigated range. It was also found that Bi₂Te₃ is a high refractive index material (n has values of 4.7–8.8 in the wavelength range 2.5–10 μm). The allowed optical transitions were found to be direct optical transitions with energy gap  eV. The optical conductivities σ₁ = ƒ(hν) and σ₂ = f(hν) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions.     

Comparative first-principles calculations of electronic, optical and elastic anisotropy properties of CsXBr3 (X=Ca,Ge,Sn ) crystals

Detailed ab initio calculations of the structural, electronic, optical and elastic properties of CsCaBr₃, CsGeBr₃ and CsSnBr₃ crystals are presented in this paper. Based on the obtained results, CsCaBr₃ is characterized as a dielectric with an indirect band gap, whereas CsGeBr₃ and CsSnBr₃ are semiconductors with very narrow direct band gaps. The first theoretical estimations of the refractive indexes for all compounds are reported. Variations of the electron density difference distribution induced by changes of the second cation were analyzed and related to the type of chemical bonding between atoms. In addition, the complete set of elastic parameters (which includes the elastic constants, elastic compliance constants, bulk and Young’s moduli, elastic anisotropy) was obtained. Directional anisotropy of elastic properties was visualized; the directions in the crystal lattices, along which the maximal and minimal values of the Young’s moduli are realized, were identified.     

First-principle study on the electronic and optical properties of Mn-doped SnO2

Using the full-potential linearized augmented plane wave method (FP-LAPW), we have investigated the electronic and optical properties of Sn_1−xMn_xO₂ (x=0, 0.0625, 0.125, 0.1875, 0.25). The doped Mn results in reduction of the band gap, which can be attributed to a series of impurity bands at the bottom of the conduction band caused by the strong hybridization between Mn 3d and O 2p. The results also show that the Mn-doped systems tend to convert into p-type semiconductor with direct band gaps. With the increase of Mn concentration, both the imaginary part of dielectric function and the absorption spectrum show red-shift corresponding to the change of band gaps.     

Influence of valence electron concentration on elastic, electronic and optical properties of the alkaline-earth tin oxides A3SnO (A=Ca, Sr and Ba): A comparative study with ASnO3 compounds

By employing first principles method of the plane wave pseudo potential calculations (PP-PW), based on the density functional theory (DFT), within the local density approximation (LDA), the correlation between valence electron concentration and structural, elastic, electronic as well as optical properties of A₃SnO and ASnO₃ compounds where A=Ca, Sr and Ba are investigated. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young's moduli for ideal monocrystalline and for polycrystalline A₃SnO and ASnO₃ aggregates. Band structures reveal that alkaline-earth tin oxides A₃SnO are direct energy band gap (G-G) materials.The hardness of these compounds was explained using chemical bonding properties and Milliken charges transfer. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 20 eV. We have found that the static dielectric constants of all these compounds are in good agreement with Penn model.     

The optical properties of Dy-doped NaY(MoO4)2 crystal

The Dy³⁺-doped NaY(MoO₄)₂ single crystals were grown successfully by the Czochralski technique. The main spectroscopic properties (absorption, luminescence, decay curve) of Dy³⁺-doped NaY(MoO₄)₂ have been determined for both the σ and π polarizations. By using the Judd-Ofelt theory, the measured room temperature absorption spectra were applied to determine the intensity parameters, spontaneous transition probabilities, branching ratios, and radiative lifetimes of Dy³⁺ transitions. The results show that the Dy³⁺-doped NaY(MoO₄)₂ crystal may realize the yellow laser operation.     

Structural and elastic properties of LaTiO3 under pressure

We investigate the structural and elastic properties of LaTiO₃ by the plane-wave pseudopotential density functional theory method. The lattice constants, bulk modulus and its pressure derivative are obtained. These properties in the equilibrium phase are well consistent with the available experimental data. The pressure dependence of the elastic constants, ductility, mechanical stabilities, sound velocity and Debye temperatures are investigated for the first time. From the ratio G/B, we conclude that LaTiO₃ is ductile at 0 GPa and becomes more ductile at high pressure. In addition, the anisotropy factors for every symmetry plane and axis as well as linear bulk modulus at diverse pressures have been obtained.