Effect of $A$-site size difference on polar behavior in $M$BiScNbO${}_6$,($M$$=$Na, K and Rb): Density functional calculations

Abstract

We investigate the effect of $A$-site size differences in the double perovskites BiScO${}_3$-$M$NbO${}_3$ ($M$$=$Na, K and Rb) using first-principles calculations. We find that the polarization of these materials is 70$\sim$90 $\hat{I}¼$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.