Investigation of the structural, electronic and dynami̇c properties of LiTaO3 crystal by Ab Initio Method

In this thesis, the structural parameters, electronic band structure, elastic coefficients, polycrystalline elastic properties, anisotropy factors, Debye temperature, phonon dispersion curve and phonon density of state of the LiTaO3 crystal in the ferroelectric and paraelectric phases were examined using ab initio method at ambient pressure. The atomic structure in both phases was optimized to obtain lattice constant, angle and atomic positions. 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. The computed single-crystal elastic coefficients indicate that mechanical stability of LiTaO3 in both phases is confirmed using the generalized 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. Some anisotropy factors and indexes were determined to investigate mechanical anisotropy. It showed weak anisotropic properties in both phases. Our results indicate that the mechanical and bonding properties of both phases are very similar. The density of phonon states and phonon dispersion curves of LiTaO3 were obtained using the direct method based on computed Hellmann–Feynman forces. All results were compared with the available theoretical computational and experimental data in the literature.