Comparative dielectric response in CaTiO3 and CaAl1/2Nb1/2O3 from first principles

CaTiO3 (CT) and CaAl1/2Nb1/2O3 (CAN) have similar structures but very different dielectric properties. For CT at room temperature, the relative permittivity kappa approximate to 170 and the temperature coefficient of resonance frequency tau (f)approximate to 900x10(-6) K-1, but for CAN, kappa approximate to 27 and tau (f)approximate to -88x10(-6) K-1. I use first-principles density functional theory calculations to investigate the origin of the contrasting behaviors. I find important differences between the two systems in both the frequencies and the eigenvectors of the low-frequency polar phonons. In CT, the frequencies are lower and the perovskite B site cations move against the surrounding oxygen octahedra; whereas in CAN, the frequencies are higher and the B site cations move with the oxygen octahedra. These two factors are equally important in explaining the differences in kappa. I introduce and solve a decoupled quantum oscillator model for the temperature-dependent permittivity. This model predicts a large positive tau (f) for CT and a small tau (f) for CAN, in qualitative agreement with experiment. I relate the different dielectric behaviors to differences in the electronic structures. (C) 2001 American Institute of Physics.