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

TL;DR

This study uses density functional calculations to explore how the size difference of $A$-site ions in double perovskites affects their polarization, revealing that larger $M$ ions enhance polarization through increased off-centering and suppressed octahedral tilts.

Contribution

It provides first-principles insights into how $A$-site size variation influences polar behavior in Bi-based double perovskites, highlighting the role of ion off-centering and octahedral tilt suppression.

Findings

Polarization reaches 70-90 μC/cm² along rhombohedral direction.

Large Bi off-centerings significantly contribute to high polarization.

Increasing $M$ ion size enhances polarization via tilt suppression and ion off-centering.

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 $\mu$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.