Cation ordering in perovskites

Although both A- and B-site cations have the same simple cubic topology in the perovskite structure they typically adopt different patterns of chemical order. As a general rule B-site cations order more readily than A- site cations. When cation ordering does occur, rock salt ordering of B/B' cations is favored in A(2)BB'X-6 perovskites, whereas layered ordering of A/A' cations is favored in AA'B2X6 and AA'BB'X-6 perovskites. The unexpected tendency for A- site cations to order into layers stems from the bond strains that would result at the anion site if A and A' cations of different size were to order with a rock salt arrangement. The bonding instabilities that are created by layered ordering are generally offset either by anion vacancies or second order Jahn-Teller distortions of a B-site cation. Novel types of A- site cation ordering can be stabilized by a(+)a(+)a(+) or a(+)a(+)c(-) tilting of the octahedra.