First-principles calculations of the dielectric properties of perovskite-type materials

We compare first-principles (FP) calculations of the ionic effective charges, phonon frequencies, and static dielectric permittivities κs of several perovskite-type materials. Transition metal ions have anomalously large effective charges, though in the double perovskite CaAl1/2Nb1/2O3 (CAN), the effective charge of Nb is significantly lower than in the simple perovskite KNbO3, showing different Nb–O bonding chemistry. Tolerance factors, cation chemistry, and structural phase transitions all affect the nature of the softest phonons in perovskites. For the solid solution (CaAl1/2Nb1/2O3)1−x–(CaTiO3)x (CAN–CT), κs is modeled via a cluster expansion, with the parameters determined from FP. In pure CAN, κs is found to increase when cation disorder increases, in agreement with experimental results on analogous systems. The dielectric constant of CAN-CT increases nonlinearly with x, in agreement with experiment.