Combined atomic ordering in the A and B sublattices of perovskite structure

The simultaneous ordering of cations in the A and B sublattices of cubic perovskite structure has been investigated by the group-theoretical method. It is found that 147 ordered phases may exist. Among them, there may be 121 phases with simultaneous cation ordering in the 1(a)- and 1(c) positions of perovskite structure. It is shown that 53 phases are described by improper order parameters related to cation ordering, the formation of 56 phases is related to improper rotations of octahedra, and 15 phases are improper ferroelectrics. Calculated structures of some types of ordered low-symmetry perovskite modifications are presented.