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Lattice Instabilities, Anharmonicity and Phase Transitions in PbTiO 3 and PbZrO 3

Most perovskite structure oxides exhibit structural phase transitions from a hightemperature cubic phase to a distorted low-temperature phase which can be described by the freezing-in of one or more phonon modes of the cubic structure [1]. The first-order cubic-tetragonal ferroelectric transition in PbTiO 3 at T c = 763 K involves the freezing-in of a single F15 polar mode. In PbZrO 3 , the structure of the antiferroelectric low-temperature orthorhombic phase is far more complicated, with forty atoms per unit cell and the freezing-in of R 25 and Σ 3 modes, perhaps accompanied by other modes as well [2][3].