FIRST-PRINCIPLES STUDY OF THE ELECTRONIC, LINEAR, AND NONLINEAR OPTICAL PROPERTIES OF Li(Nb, Ta)O-3

WOS: 000288714800006We investigate the energy band structure, total density of states, the linear, nonlinear optical (NLO) response, and the electron energy-loss spectrum for Li(Nb, Ta)O-3 using first principles calculations based on density functional theory in its local density approximation. Our calculation shows that these compounds have similar structures. The indirect band gaps of 3.39 eV(LiNbO3) and 3.84 eV (LiTaO3) at the Gamma Z direction in the Brillouin zone are found. A simple scissor approximation is applied to adjust the band energy gap from the calculations to match the experimental values. The optical spectra are analyzed and the origins of some of the peaks in the spectra are discussed in terms of calculated electronic structure. Calculations are reported for the frequency-dependent complex second-order NLO susceptibilities chi((2))(ijk)(-2 omega, omega, omega) up to 10 eV and for zero-frequency limit chi((2))(ijk)(0). The results are compared with the theoretical calculations and the available experimental data

Electronic structure and optical properties of KNbO3: First principles study

WOS: 000253707500003The electronic structures and optical properties of KNbO3 were studied from the first principles using density functional theory. The energy band structures, dielectric function and optical constants of cubic and tetragonal phases are calculated using pseudopotential with local density approximation. A direct band gap at G point and an indirect band gap R- G point in the Brillouin zone are predicted for cubic and tetragonal phases. Both phases, the real and imaginary parts of the dielectric function and hence the optical constants such as reflectivity, refractive index, extinction coefficient, electron energy-loss function, E-eff (the optical dielectric constant) and N-eff (the effective number of electrons) per unit cell are calculated. The calculated spectra are compared with the experimental data in the orthorhombic phase and other theoretical results for KNbO3 and are found to be in good agreement with the results

Ab initio study of the linear and nonlinear optical responses in BiAlO3

WOS: 000369872800006The linear and non-linear optical properties of BiAlO3 are studied by employing the density functional perturbation theory within the local density and generalized gradient approximations. The computations are based on the electronic structure obtained within density functional theory. The optical properties such as the dielectric function, refractive index, spectral reflectivity, absorption coefficient and electron energy-loss spectrum are obtained in the energy region of up to 30 eV. The calculated value of the birefringence for BiAlO3 shows that it is a uniaxial negative crystal and has a large birefringence. We also report our studies on the second harmonic generation response coefficient over a large frequency range for BiAlO3 crystal. The predicted second-order optical spectra indicate pronounced structures related to of 1 and 2 frequency resonances. Furthermore, the non-linear optic and linear electro-optic coefficients are computed by employing 2n+1 theorem applied to an electric-field dependent energy functional. The results are compared with the available calculations.Cukurova UniversityCukurova University [FEF2013BAP20]This research is supported by Cukurova University [project number FEF2013BAP20]

First-principles studies of the electronic structure and optical properties of AgBO3 (B=Nb,Ta) in the paraelectric phase

WOS: 000257770700045The electronic energy-band structure, density of states (DOS), and optical properties of AgBO3 in the paraelectric cubic phase have been studied by using density functional theory within the local density approximation for exchange-correlation for the first time. The band structure shows a band gap of 1.533 eV (AgNbO3)and 1.537 eV (AgTaO3)at (M-I-)point in the Brillouin zone. The optical spectra of AgBO3 in the photon energy range up to 30 eV are investigated under the scissor approximation. The real and imaginary parts of the dielectric function and - thus the optical constants such as reflectivity, absorption coefficient, electron energy-loss function, refractive index, and extinction coefficient - are calculated. We have also made some comparisons with related experimental and theoretical data that is available

Ab initio study of the lattice dynamical and thermodynamic properties of SbXI (X = S, Se, Te) compounds

WOS: 000444502900004The Born effective charges, high-frequency dielectrics constants, vibrational and thermodynamics properties of paraelectric SbSeI, SbTeI, SbSI and ferroelectric SbSI phase have been calculated by ab-initio method within the local density approximation. The high-frequency dielectric and Born effective charge tensor components are analyzed within density functional perturbation theory. We find remarkably enhanced/reduced Born effective charges and anomalously high-frequency dielectric constants for SbXI compounds. The phonon dispersion spectra and density of phonon states of these compounds were acquired using the direct method based on calculated Hellmann-Feynman forces. A comparison with the phonon dispersion curves of SbXI compounds displays minor differences that can attributed to the atomic-mass difference among S, Se and Te. The Raman and infrared active phonon mode frequencies of compounds are predicted at the Gamma point. The thermodynamic properties of these compounds such as the internal energy, Helmholtz free energy, entropy, heat capacity, and Debye temperature are also investigated and discussed in the framework of the harmonic approximation. The obtained results were compared with available experimental and other theoretical data. (C) 2018 Published by Elsevier B.V.Cukurova UniversityCukurova University [FEF2011D29]This work is supported by Cukurova University under Project No. FEF2011D29. The authors would like to thank Prof. Dr. Metin Ozdemir for the English technical support

DFT - based study of electronic structures and mechanical properties of LiTaO3 : ferroelectric and paraelectric phases

WOS: 000407248600001By applying ab initio calculation within density functional theory (DFT), we study the structure parameters, electronic band structure, elastic coefficients, polycrystalline elastic properties, anisotropy factors and Debye temperature of ferroelectric and paraelectric phases of LiTaO3 within the generalised gradient approximation at ambient pressure. The atomic structure in both phases is fully relaxed and the lattice constant, angle and atomic positions are well consistent with experimental values. The computed single-crystal elastic coefficients indicate that mechanical stability of LiTaO3 in both phases is confirmed using the generalised Born criteria. The shear, bulk and Young's modulus, Poisson's ratio, and Vickers hardness were computed according to theoretical elastic constants by Voight-Reuss-Hill method. Several anisotropy factors and indexes are computed to illustrate mechanical anisotropy. Both phases are shown to be weakly anisotropic. The Debye temperature is estimated using the longitude and transverse elastic wave velocity of the ideal polycrystalline LiTaO3 aggregates. We have found that LiTaO3 in both phases has an indirect energy band gap. The differences in the electronic structure and density of states for both phases are quite small. Our results indicate that the mechanical and bonding properties of both phases are very similar. The obtained results were compared with the available experimental and theoretical values.Cukurova UniversityCukurova University [FEF2012YL16]This work was supported by Research Fund of the Cukurova University [grant number FEF2012YL16]

First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O-3

WOS: 000316813600014Electronic and optical properties of Sr(Ti,Zr)O-3 crystals in the cubic (Pm-3m) and tetragonal (I4/mcm) phase were calculated by the first-principles calculations using the density functional theory and the local density approximation. The band structure of cubic and tetragonal phases show an indirect band gap at (R-I") point and at (M-I") point in the Brillouin zone, respectively. The linear photon-energy dependent dielectric functions and some optical properties such as the absorption coefficient, energy-loss function and reflectivity are calculated for both phases. The optical properties of tetragonal phase of Sr(Ti,Zr)O-3 were investigated by theoretical methods for the first time. We have also made some comparisons with the available related experimental and theoretical data.Cukurova UniversityCukurova University [FEF2006YL72]This work was supported by Cukurova University under project number FEF2006YL72. The authors are grateful to the ABINIT group for their ABINIT project that we used in our computations

Ab initio study of the structural, electronic and optical properties of NaTaO3

WOS: 000278314100007Ab initio calculations were performed of the structural, electronic and optical properties of the cubic ([image omitted], tetragonal ([image omitted], and orthorhombic ([image omitted]) phases of NaTaO3 using a plane-wave pseudopotential method within the density-functional theory. Results are presented for the structural properties, electronic band structure, density of states and imaginary and real parts of the frequency-dependent linear optical response. Cubic and tetragonal NaTaO3 both have an indirect band gap, at the R- and Z- points, respectively, whereas, orthorhombic NaTaO3 has a direct band gap at the - point. The optical properties of NaTaO3 were investigated by ab initio calculation under the scissor approximation. The real and imaginary parts of the dielectric function and, hence, the optical constants (such as absorption coefficient and the electron energy-loss spectrum) were calculated. This is the first quantitative theoretical prediction of optical properties, except for absorption of the orthorhombic phase, of the NaTaO3 compound

Structural, electronic, elastic and vibrational properties of BiAlO3: A first principles study

WOS: 000321749600084We present a study of the structural, electronic, elastic and vibrational properties of the rhombohedral BiAlO3 structure within the local density approximation of density functional theory using norm-conserving pseudopotentials. The calculated equilibrium lattice constant, angle and atomic position are in reasonable agreement with the available experimental and theoretical dates. Based on the elastic constants and their related parameters, the crystal mechanical stability have been discussed. The elastic constants for BiAlO3 are also needed to completely determine its elastic properties including polycrystalline bulk, shear and Young's moduli, Poisson's ratio and the elastic anisotropy. Energy band structure shows that the rhombohedral BiAlO3 has an indirect band gap between D and U-D symmetry points. We compute Born effective charge tensor, which is found to be quite anisotropic of Bi and O atoms. BiAlO3 have been studied by applying the direct method and deriving the phonon dispersion relations which include the longitudinal/transverse optical phonon mode splitting. In the rhombohedral phase the phonon dispersion curves show a soft mode between X and U-point. This soft mode leads to the observed rhombo-hedral-cubic phase transition. The results are compared with the previous calculations and available experimental data. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.Cukurova UniversityCukurova University [FEF2011D13, FEF2010BAP8]This work supported by Cukurova University under Project Number FEF2011D13 and FEF2010BAP8. The authors are grateful to SIESTA group that we used in our computations