Polar behavior of double perovskite (Bi,Pb)ZnNbO6 and (Bi,Sr)ZnNbO6: Density functional calculations

TL;DR

This study uses density functional calculations to explore the ferroelectric properties of double perovskites (Bi,Pb)ZnNbO$_6$ and (Bi,Sr)ZnNbO$_6$, revealing strong polarization mainly due to Bi off-centering and Nb charge effects.

Contribution

It provides first-principles insights into the polar behavior and potential phase boundaries of these double perovskites, highlighting their high polarization and structural tendencies.

Findings

High polarization (>80 μC/cm²) in Pb-based material.

Bi off-centering and Nb effective charge drive ferroelectricity.

No preference for tetragonal phase, suggesting possible morphotropic boundaries.

Abstract

The polar behavior of double perovskite (Bi,Pb)ZnNbO$_6$ and (Bi,Sr)ZnNbO$_6$ was investigated using first principles density functional calculations within the local density approximation. These materials have both $A$-site size disorder and ions with sterochemical activity. We found a strong ferroelectric distortion in both materials. The polarization of the Pb material is $>$80 $\mu$C/cm$^2$ along the rhombohedral direction and the Sr based material is only slightly inferior. The high polarization results mainly from a large off-centering of Bi and the large Born effective charge of Nb, which yields a large contribution polarization although the Nb off-centers by a smaller amount. Neither of these materials favors a tetragonal state, and therefore solid solutions with PbTiO$_3$ may show morphotoropic phase boundaries.