Effect of A-site size difference on polar behavior in MBiScNbO6 (M=Na, K, and Rb): Density functional calculations

We investigate the effect of A-site size differences in the double perovskites BiScO3-MNbO3 (M= Na, K, and Rb) using first-principles calculations. We find that the polarization of these materials is 70-90 mu C/cm(2) 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.