Investigatin with Ab initio method of dynamic properties of ABO3 (A=Ag, Na ; B=Nb, Ta) type crystals

In this dissertation, we have investigated the structural, electronic and dynamical properties of the AgNbO3, NaTaO3, NaNbO3 in the orthorhombic structures and AgTaO3 in the rhombohedral structure, by employing the plane wave pseudopotential method, local density approximation of the density functional theory for the first time. The calculated equilibrium lattice parameters and atomic position are in the agreement with the available experimental and theoretical results. AgNbO3 and AgTaO3 both have an indirect band gap, whereas, NaTaO3 and NaNbO3 both have a direct band gap in the Brillouin zone. Born effective charges, dielectric permittivity tensors and elastic properties have been calculated within the framework of density functional perturbation theory. Born effective charges tensors for each ion, which determine the non-analytic contribution to the dynamical matrix in the limit NaTaO3 and NaNbO3 q?0. The phonon dispersion curves and the phonon density of states are derived using the direct method. The LO / TO splitting was extracted from phonon frequencies found within the supercell. Gamma point phonon calculations give negative frequencies which indicates the AgNbO3, NaTaO3, NaNbO3 and AgTaO3 structures are metastable. We have also made comparisons with related experimental and theoretical data which is available.