Polar behavior of the double perovskites BiM ZnNbO₆ ( M=Pb and Sr) from density-functional calculations

The polar behavior of double perovskite BiPb ZnNbO₆ and BiSr ZnNbO₆ 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 μC/ cm² 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 PbTiO₃ may show morphotropic phase boundaries.