Electronic structure calculations of transparent conductor Cd3TeO6: Optical properties and the effect of In-substitution

The electronic structure of Cd₃TeO₆ has been studied in the terms of first-principles calculations based on the density functional theory in order to investigate their optical properties and In-substitution effects. It was found that the highly dispersed bottom of the conduction band formed from Cd-s orbitals is the origin of the high transparency and conductivity. Cd₃TeO₆ exhibited optical anisotropy in its main crystal axes, and the c-axis showed the most suitable crystal growth direction for obtaining a wide transparent region. A pronounced shift of the absorption edge was effectively observed by the In-substitution, reflecting the domination of the In-5s level in the conduction band near the Fermi level.     

First-principles calculations of the electronic and optical properties of In6S7 compound

The electronic structure and the optical properties of In₆S₇ crystal are calculated by the first-principles full-potential linearized augmented plane wave method (FP-LAPW) using density functional theory (DFT) in its generalized gradient approximation (GGA). The calculated band structure shows that the In₆S₇ is a semiconductor with a direct band gap in good agreement with experimental studies. Furthermore, the dielectric tensor and the optical properties, such as absorption coefficient, refractive index, extinction coefficient, energy-loss spectrum and reflectivity, are derived and analyzed in the study.     

Theoretical Prediction for Structural Stabilities and Optical Properties of SrS, SrSe and SrTe under High Pressure

An investigation on the structural stabilities and electronic properties of SrX （X =S, Se and Te） under high pressure is conducted using the first-principles calculation based on density functional theory （DFT） with the plane wave basis set as implemented in the CASTEP code. Our results demonstrate that the sequence of the pressure-induced phase transition of the three compounds is the NaCl-type （B1） structure （Fm3rn） to the CsC1- type （B2） structure （Pm3m）. The phase transition and the metallization pressures are determined theoretically. The pressure effect on the optical properties is discussed. The results are compared with the previous calculations and experimental data.     

First-Principles Study of Structural Stabilities, Electronic and Optical Properties of SrF2 under High Pressure

An investigation of structural stabilities, electronic and optical properties of SrF2 under high pressure is conducted using a first-principles calculation based on density functional theory （DFT） with the plane wave basis set as implemented in the CASTEP code. Our results predict that the second high-pressure phase of SrF2 is of a Ni2In- type structure, and demonstrate that the sequence of the pressure-induced phase transition of SrF2 is the fluorite structure （Fm3m） to the PbC12-type structure （Pnma）, and to the Ni2In-type phase （P63/mmc）. The first and second phase transition pressures are 5. 77 and 45.58 GPa, respectively. The energy gap increases initially with pressure in the Fm3m, and begins to decrease in the Pnma phases at 30 GPa. The band gap overlap metallization does not occur up to 210 GPa. The pressure effect on the optical properties is discussed.     

Effect of In-Doping on Electronic Structure and Optical Properties of Sr2TiO4

The effect of In doping on the electronic structure and optical properties of Sr2 TiO4 is investigated by a firstprinciples calculation of plane wave ultrasoft pseudopotentials based on density functional theory. The calculated results reveal that corner-shared TiO6 octahedra dominate the main electronic properties of Sr2TiO4 and the covalency of the Ti-O（1） bond in the ab plane is stronger than that of the Ti-O（2） bond along the c-axis. After In doping, there is a little lattice expansion in Sr2In0.125 Ti0.875 O4 and the interaction between the Ti-O bond near the impurity In atom is weakened. The binding energies of Sr2TiO4 and Sr2In0.125Ti0.875O4 estimated from the electronic structure calculations indicate that the crystal structure of Sr2In0.125 Ti0.875 O4 is still stable after doping, but its stability is lower than that of undoped Sr2TiO4. Moreover, the valence bands （VBs） of the Sr2In0.125Ti0.875O4 system consist of O 2p and In 4d states, and the mixing of O 2p and In 4d states makes the top VBs shift significantly to high energies, resulting in visible light absorption. The adsorption of visible light is of practical importance for the application of St2 TiO4 as a photocatalyst.     

Electronic properties and elastic constants of wurtzite, zinc-blende and rocksalt AlN

Electronic properties and elastic constants of AlN in the wurtzite, zinc-blende and rocksalt structures are investigated using an ab initio pseudopotential method based on the density-functional theory with both the local-density approximation and the generalized gradient approximation for the exchange-correlation functional. The numerically calculated results compare well with the existing experimental data. For elastic constants of rocksalt AlN our results are predictions.     

First principles study of electronic and optical properties of zincblende InP

Electronic and optical properties of InP in zincblende crystal structure are studied using the full potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) program. The complex dielectric function and optical constants, such as optical absorption coefficient, reflectivity, refractive index and extinction coefficient, are calculated, which are in good agreement with the experimental results, better than those from the full potential linear muffin-tin orbital (FP-LMTO) method. We also have explained the origin of the spectral peaks on the basis of the electronic band structures.     

First-principles study of electronic structure, chemical bonding, and optical properties of cubic SrHfO3

Electronic structure and optical properties of SrHfO₃ are calculated using the full potential linearized augmented plane wave plus local orbitals method. The calculated equilibrium lattice is in reasonable agreement with the experimental data. From the density of states (DOS) as well as charge density studies, we find that the bonding between Sr and HfO₃ is mainly ionic and that HfO₃ entities bond covalently. The complex dielectric functions are calculated, which are in good agreement with the available experimental results. The effect of the spin-orbit coupling on the optical properties is also investigated and found to be quite small.     

Ab initio study of electronic structures and magnetism in ZnMnTe and CdMnTe diluted magnetic semiconductors

In this work, we aimed to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of ZnMnTe- and CdMnTe-diluted magnetic semiconductors (DMSs) in the ferromagnetic phase, and with 25% of Mn. The calculations are performed by the recent ab initio full potential augmented plane waves plus local orbitals (FP−L/APW+lo) method within the spin-polarized density-functional theory and the local spin density approximation. We have determined the exchange splittings produced by the Mn d states: Δ_x(d) and Δ_x(pd), and we found that the effective potential for the minority spin is more attractive than that for the majority spin. Also, we show the nature of the bonding from the charge spin-densities calculations, and we calculate the exchange constants N₀α and N₀β, which mimics a typical magneto-optical experiment. The calculated total magnetic moment is found to be equal to 5μ_B for both DMSs. This value indicates that every Mn impurity adds no hole carriers to the perfect ZnTe and CdTe crystals. Furthermore, we found that p–d hybridization reduces the local magnetic moment of Mn and produces small local magnetic moments on the nonmagnetic Te, Zn and Cd sites.     

Theoretical and experimental study on the electronic structure and optical absorption properties of P-doped TiO2

Phosphorus-doped nanosized TiO₂ powders were prepared by a sol-gel technology. The optical absorption studies revealed that the spectral responses of phosphorus-doped (P-doped) TiO₂ powders shift to the visible light region. The optimum phosphorus (P) content in our experiments is 16.7% (mol), and the corresponding absorption edge shifts to 450 nm. Furthermore, our ab initio calculations support the conclusion that the doping of phosphorus can reduce the band gap by mixing the P 3p states with O 2p states. The theoretical lattice parameters and optimum phosphorus content are in agreement with the experimental results.     

Structural and Thermodynamic Properties of Gallium Arsenide with Hexagonal Wurtzite Structure from First-Principles Analysis

A first-principles plane wave method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation （GGA） is performed to calculate the lattice parameters, the bulk modulus Bo and its pressure derivative B~o of the hexagonal wurtzite GaAs （w-GaAs） by the Cambridge serial total energy package （CASTEP）. Our calculations show that the most stable structure of the w-GaAs corresponds to the axial ratio c/α = 1.651 and the internal parameter u = 0.374, consistent with other theoretical results. Also, the thermodynamic properties of the w-GaAs are investigated from the quasi-harmonic Debye model. The dependences of the normalized lattice parameters α/α0, c/c0, the axial ratio c/α, the normalized volume V/V0, the heat capacity Cv and the thermal expansion α on pressure P and temperature T are also obtained successfully.     

Prediction study of elastic properties under pressure effect for filled tetrahedral semiconductors LiZnN, LiZnP and LiZnAs

First principle calculations of elastic properties under pressure of the filled tetrahedral semiconductors LiZnN, LiZnP and LiZnAs are presented, using the pseudo-potential plane-waves approach based on density functional theory, within the local density approximation. Elastic constants, bulk modulus, Young’s modulus and Poisson’s ratio are calculated at zero pressure. A linear dependence of the bulk modulus and elastic constants with applied pressure is found. As the experimental elastic constants are not available for LiZnX, we have also calculated the elastic constants of GaN, GaP and GaAs, the binary analogues of LiZnN, LiZnP and LiZnAs, respectively, for checking the reliability and accuracy of our predicted results for LiZnX. The obtained results agree well with the available experimental data.     

Electronic Structure and Optical Properties of Semiconducting Orthorhombic BaSi2

Full potential linearized augmented plane wave （FPLAPW） method calculations are carried out for semiconducting orthorhombic BaSi2. The optical properties and the origin of the different optical transitions are investigated. Our calculated band gap of 1.0918eV is indirect, which is in good agreement with the experimental result. The bonds between Ba and Si are considered to be electrovalent bond. The anlsotropy in the imaginary part ε2（w） and real part εl（w） of the optical dielectric tensor are analysed. The contributions of various transition peaks are explained from the imagnary part of the dielectric function.     

First-principles calculations of structural, elastic, electronic and optical properties of the antiperovskite AsNMg3

The density functional theory (DFT) calculations of structural, elastic, electronic and optical properties of the cubic antiperovskite AsNMg₃ has been reported using the pseudo-potential plane wave method (PP-PW) within the generalized gradient approximation (GGA). The equilibrium lattice, bulk modulus and its pressure derivative have been determined. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus and Poisson's ratio for ideal polycrystalline AsNMg₃ aggregate. We estimated the Debye temperature of AsNMg₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of AsNMg₃ compound, and it still awaits experimental confirmation. Band structure, density of states and pressure coefficients of energy gaps are also given. The fundamental band gap (Γ-Γ) initially increases up to 4 GPa and then decreases as a function of pressure. Furthermore, the dielectric function, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. The all results are compared with the available theoretical and experimental data.     

Structural phase transition of boron nitride compound

The full-potential band-structure scheme based on the linear combination of overlapping nonorthogonal local-orbital (FPLO) is used. The crystal potential and density are represented as a lattice sum of local overlapping nonspherical contributions. The energetic transitions of BN of zinc-blende and wurtzite structures are calculated using the band structure scheme. The energy gap at ambient pressure is found to be indirect for the two structures. The structural properties of two structures of BN are (obtained from the total energy calculations) and the total density of states are calculated. The phase transition parameter of BN is investigated. The ionicity character of BN has been calculated to test the validity of our recent models. The results are in reasonable agreement with experimental and other theoretical results.     

First-principle calculations of structural, electronic and optical properties of BaTiO3 and BaZrO3 under hydrostatic pressure

A theoretical study of structural, electronic and optical properties of cubic BaTiO₃ and BaZrO₃ perovskites is presented, using the full-potential linear augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus, its pressure derivative, band structure, density of states, pressure coefficients of energy gaps and refractive indices. The results are compared with previous calculations and experimental data.     

First principles study of structural, elastic, electronic and optical properties of the cubic perovskite BaHfO3

First principles study of structural, elastic, electronic and optical properties of the cubic perovskite-type BaHfO₃ has been reported using the pseudo-potential plane wave method within the local density approximation. The calculated equilibrium lattice is in a reasonable agreement with the available experimental data. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear pressure dependence of the elastic stiffnesses is found. Band structures show that BaHfO₃ is a direct band gap between the occupied O 2p and unoccupied Hf d states. The variation of the gap versus pressure is well fitted to a quadratic function. Furthermore, in order to understand the optical properties of BaHfO₃, the dielectric function, absorption coefficient, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. We have found that O 2p states and Hf 5d states play a major role in the optical transitions as initial and final states, respectively. This is the first quantitative theoretical prediction of the elastic, electronic and optical properties of BaHfO₃ compound, and it still awaits experimental confirmation.     

Electronic structure calculations for BaSxSe1−x alloys

A series of first principles calculations have been carried out to study structural, electronic properties of BaS_xSe_1−x alloys. We have used the local density as well as the generalized gradient approximations for the exchange-correlation potential. The structural properties of these materials, in particular the composition dependence to the lattice constant and bulk modulus, are found to be linear. It is also found linear relationship between theoretical band gaps and 1/a² (where a is lattice constant).     

Electronic Structure and Optical Properties of La-Doped SrTiO3 and Sr2TiO4 by Density Function Theory

The effect of La doping on the electronic structure and optical properties of SrTiO₃ and Sr₂TiO₄ is investigated by the first-principles calculation of plane wave ultrasoft pseudopotential based on the density function theory (DFT). The calculated results reveal that the electron doping in the case of Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ can be described within the rigid band model. The La³⁺ ions fully acts as electron donors in Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ systems and the Fermi level shifts further into the conduction bands (CBs) for Sr_1.875La_0.125TiO₄ compared to Sr_0.875La_0.125TiO₃. The two systems exhibit n-type degenerate semiconductor features. At the same time, the density of states (DOS) of the two systems shift towards low energies and the optical band gaps are broadened. The Sr_1.875La_0.125TiO₄ is highly transparent with the transmittance about 90% in the visible range, which is larger than that of Sr_0.875La_0.125TiO₃(85%). The wide band gap, small transition probability and weak absorption due to the low partial density of states (PDOS) of impurity in the Fermi level result in the optical transparency of the films...     

Optical functions of chalcopyrite CuGaxIn1-xSe2 alloys

We report pseudodielectric functions <ε> of the quaternary semiconductor alloys CuGa_xIn_1-xSe₂. Measurements were done in polycrystalline samples from 0.7 to 5.2 eV at room temperature by spectral ellipsometry. Accurate values of refractive indices n and extinction coefficients k representative of the bulk materials are obtained from the data. The structures observed in <ε> are analyzed by fitting the numerically differentiated spectra of d²<ε>(ω)/dω² to analytic line shapes. The obtained energies are related to given inter-band transitions by taking into account the electronic band structures of the ternary end-point compounds. Received: 20 December 2000 / Accepted: 30 May 2001 / Published online: 25 July 2001