Effect of A -site size difference on polar behavior in M BiScNbO ₆ (M=Na, K, and Rb): Density functional calculations

We investigate the effect of A -site size differences in the double perovskites BiScO₃-M NbO₃ (M=Na, K, and Rb) using first-principles calculations. We find that the polarization of these materials is 70-90 μC/cm² along the rhombohedral direction. The main contribution to the high polarization comes from large off-centerings of Bi ions, which are strongly enhanced by the suppression of octahedral tilts as the M -ion size increases. A high Born effective charge of Nb also contributes to the polarization and this contribution is also enhanced by increasing the M -ion size.