Suleyman Cabuk

Urbach rule and optical properties of the LiNbO3 and LiTaO3

Optical spectroscopy methods are used to study the spectra of some oxygen-octahedral ferroelectrics - namely LiNbO3 and LiTaO3. It is shown that for the measured range of E and T, varies exponentially with the photon energy following the empirical Urbach rule. A study of the role of the BO6-octahedron by using spectroscopic methods can cast light on the many physical phenomena that take place in LiNbO3 and LiTaO3. For these crystals, absorption edge, band gap, Urbach constants and phonon energy are found.

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

The 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-⌈)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.

The nonlinear optical susceptibility and electro-optic tensor of ferroelectrics: first-principle study

The nonlinear optical properties of some ABO3 materials (BaTiO3, KNbO3, LiTaO3 and LiNbO3) are studied by density functional theory (DFT) in the local density approximation (LDA) expressions based on first-principle calculations. Our goals are to give the details of the calculations for linear and nonlinear optical properties, including the linear electro-optic (EO) tensor for some ABO3 structures with oxygen octahedral structures using first-principles methods. These results can then be used in the study of the physics of ferroelectrics, specifically, we present calculations of the second harmonic generation response coefficient Xijk(2) (−2ω, ω, ω) over a large frequency range for ABO3 crystals. The electronic linear EO susceptibility Xijk(2) (−ω, ω,0) is also evaluated below the band gap. These results are based on a series of the LDA calculations using DFT. Results for Xijk(2) (−ω, ω,0) are in agreement with experiments below the band gap. The results are compared with the theoretical calculations and the available experimental data.

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

Ab initio calculations were performed of the structural, electronic and optical properties of the cubic ( , tetragonal ( , and orthorhombic ( ) 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–Γ and Z–Γ points, respectively, whereas, orthorhombic NaTaO3 has a direct band gap at the Γ–Γ 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.

First-principles calculation of the linear and nonlinear optical properties of LiTaO3

The linear and nonlinear optical (NLO) properties of the paraelectric and ferroelectric (FE) phases of lithium tantalate crystals were calculated using a first-principles approach based on density functional theory with the generalized gradient approximation. We present our results for the structural parameters, the imaginary and real parts of the frequency-dependent linear optical response, optical functions such as the spectral reflectivity, the absorption coefficient and the electron energy-loss spectrum. A simple scissors operator is applied to adjust the energy gap from calculations to match the experimental value. In the FE phase, we also study the NLO susceptibilities and calculate the NLO susceptibility tensor. LiTaO3 displays a good NLO effect. The results are compared with the theoretical calculations and available experimental data.

First-principles study of the structural, elastic and electronic properties of SbXI (X=S, Se, Te) crystals

The structural, elastic, elastic anisotropy and electronic properties of ferroelectric SbSI and paraelectric SbSI, SbSeI and SbTeI crystals were computed using the local density approximation with first-principle calculations, based on density functional theory. The independent elastic constants of SbXI compounds were computed and the results reveal that they are mechanically stable. Some polycrystalline quantities such as bulk modulus, shear modulus, acoustic velocities, Young’s modulus, Poisson’s ratio, elastic anisotropy and elastic Debye temperatures of these compounds were derived from computed elastic constants. Energy band structures show that these compounds have an indirect band gap. The electronic charge distribution and partial density of states of SbXI compounds indicate that the Sb-X bond is typically covalent with a strong hybridization as well as Sb-I compounds that have strong ionic character. The results obtained were compared with experimentally measured values and other theoretical data.

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

Abstract The 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 1ω and 2ω 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.

First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O3

Electronic and optical properties of Sr(Ti,Zr)O3 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-Γ) point and at (M-Γ) 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)O3 were investigated by theoretical methods for the first time. We have also made some comparisons with the available related experimental and theoretical data.

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

Abstract By 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.

Investigation of Mechanical Properties of SnO2 Crystals in CaCl2 Type Structure

Anahtar kelimeler VASP; DFT; SnO2; Bulk Modül; Debye Sıcaklığı Özet Mekanik özellikler malzeme bilimi ve teknolojisinde kilit rol oynamaktadır. Mekanik özellikler, değişik basınç altında yoğunluk fonksiyonel teorisine dayanan ab-initio yöntemi ile genelleştirilmiş gradyent yaklaşımı (GGA) kullanılarak hesaplanmıştır. CaCl2 fazındaki SnO2’nin yapısal parametreleri belirlendi. Bağımsız elastik kat sayıları ve bulk modülü, Debye sıcaklığı gibi ilgili polikristal nicelikler Voight -ReussHill yaklaşımları kullanılarak hesaplandı. Ortam basıncında Debye sıcaklığı ve x, y ve z yönlerinde ortalama lineer sıkıştırılabilirlik sırasıyla 487,1K ve 1,89 TPa -1 olarak bulundu. Ayrıca, farklı basınçlarda örgü parametreleri ve elastik sabitler üzerinde regresyon analizi yapıldı ve basınca bağlı olarak regresyon eşitlikleri önerildi. Elde edilen sonuçlar önceki hesaplamalar ve mevcut deneysel verilerle karşılaştırılmıştır.