The polar behavior of double perovskite $\text{Bi}\text{Pb}{\text{ZnNbO}}_6$ and $\text{Bi}\text{Sr}{\text{ZnNbO}}_6$ was investigated using first-principles density-functional calculations within the local-density approximation. These materials have both $A$-site size disorder and ions with stereochemical activity. We found a strong ferroelectric distortion in both materials. The polarization of the Pb material is $>80\mu \text{C}/{\text{cm}}^2$ 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 to the polarization although the Nb off-centers by a smaller amount. Neither of these materials favors a tetragonal state, and therefore solid solutions with ${\text{PbTiO}}_3$ may show morphotropic phase boundaries.

• Received 13 January 2010

DOI:https://doi.org/10.1103/PhysRevB.81.134106