Theory of Symmetry Change in Second-Order Phase Transitions in Perovskite Structure

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Abstract
An analysis is presented of the compatible subgroups which can arise by second-order phase transition from the perovskite structure, without change of unit cell. The symmetry analysis is based upon the subduction-criterion simplification of the thermodynamic Landau theory. The possible displacements from the perovskite structure are taken as the normal modes at k=(0, 0, 0). Additional criteria for selecting compatible subgroups are developed, based on a chain of subgroups. Only eight space groups can arise in this manner, of which six can be ferroelectric. An error in the previous theoretical analysis by Haas is eliminated. Our results are compared to the meager experimental determinations of compatible subgroups, but more such determinations are needed.