Cooperative behavior of Zn cations in Bi-based perovskites: A comparison of (Bi,Sr)2ZnNbO6 and (Bi,Sr)2MgNbO6

We investigated the polar behavior of the double perovskite (Bi, Sr)(2)MgNbO6 using first-principles density-functional theory calculations. We find that the magnitude (75 mu C/cm(2)) and direction (along [ 111]) of the polarization are comparable to our previous results for the A-site size difference (Bi, Sr)(2)ZnNbO6 and (Bi, Pb)(2)ZnNbO6 systems. However, comparisons with the (Bi, Sr)(2)ZnNbO6 compound indicate that the presence of Zn modestly enhances the off-centering of the Sr and Nb cations as well as the Born effective charges of both Bi and Nb. Analogous to the corresponding Pb-based perovskites, Pb(Mg1/3Nb2/3)O-3 and Pb(Zn1/3Nb2/3)O-3, we demonstrate that the differences in the experimentally observed critical temperatures are related to the differences in polarization between the two materials. A local dipole analysis indicates that the most significant contribution arises from the enhanced cooperative couplings with the larger Zn displacements.