Displacive phase transitions and intermediate structures in perovskites

Pb-based, complex perovskites and compositions at phase boundaries in solid solutions exhibit intermediate structures between established displacive variants. The most high-profile example is the intermediate monoclinic structure in Pb(Zr_xTi_1-x)O₃ (PZT), which occurs because of frustration between displacive variants with different polarization vectors. However, examples are also given in which intermediate structures may also be stabilized in solid solutions between displacive variants whose order parameters are dominated by strain, amplitude of octahedral rotation, or combinations thereof. In solid solutions such as PZT, the frustration is compositionally driven across the phase boundaries. However, in complex perovskites such as PbSc_1/2Ta_1/2O₃, PbCo_1/2W _1/2O₃, and PbIn_1/2Nb_1/2O ₃, the type of intermediate structure is related to the degree of B-site cation order, which may be modified by thermal treatment. Depending on the degree of order and the size difference of the B-site cations, either incommensurate modulations, mixed antipolar/polar order, or antipolar order may be observed.