Investigation of the structural, electronics and dynamical properties of BiXO3 (X:Al, Co, Cr, Sc, Y)compounds by AB initio

In this dissertation, the structural, electronic, Born effective charges, elastic and vibrational properties of tetragonal BiCoO3, rhombohedral BiAlO3 and BiYO3, and monoclinic BiCrO3 and BiScO3 crystals are investigated by using ab initio calculation method within norm-conserving pseudopotentials and density functional theory. The calculated equilibrium lattice constants, angles and atomic positions are in reasonable agreement with the available experimental and theoretical datas. Energy band structures show that BiCoO3; BiAlO3; BiYO3 and BiScO3 have an indirect band gap. BiCrO3 crystal has an indirect band gap for down-spin, whereas a band gap for upspin is not observed. The density of states and Born effective charges tensor analysis show that X-O and Bi-O bands are in covalent bonding with a strong hybridization. The elastic constants were obtained using ?volume-conserving? technique show that BiXO3 crystals are mechanically stable. The phonon dispersion curve for BiXO3 crystals was derived using the ?direct method?. The longitudinal/transverse optical phonon modes splitting are estimated by calculating phonons from the supercell. The phonon dispersion curves for BiXO3 crystals show the soft modes which lead from the phase to other phase transition.