First-principles electronic structure and lattice dynamics of the giant dielectric compound Na0.5Bi0.5Cu3Ti4O12

We report the results of an ab initio study of electronic, dielectric, and lattice dynamical properties of high dielectric constant perovskite-derived oxide Na1/2Bi1/2Cu3Ti4O12. The calculations have been carried out within the local spin density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. The ground state is found to be an antiferromagnetic direct band gap semiconductor. Lattice dynamical properties, such as Born effective charge tensors, dielectric permittivity tensors, and phonon frequencies at the Brillouin zone center were calculated using density functional perturbation theory and found to be similar to the more studied CaCu3Ti4O12 and CdCu3Ti4O12 compounds. The calculated electronic (epsilon(infinity) approximate to 11.5) and static (epsilon(0) approximate to 150) dielectric constants indicate that the observed high dielectric constant is extrinsic in origin. The main contribution to the static dielectric constant is found to be due to a low frequency (50 cm(-1)) infrared-active mode which has a large mode effective charge.