Ferroelastic phase transitions in lead phosphate–vanadate Pb3(PxV1−xO4)2

Abstract

The 'pure' ferroelastic behaviour of P-rich Pb3(PxV1 - xO4)2 is of the improper type. The ferroelastic phase transition is antiferrodistortive and accompanied by a second one at a lower temperature. The temperature range of the intermediate phase is approximately 20°C. At the transition points singularities of phonon spectra, dielectric and optical behaviour, and lattice constants are found. The Raman frequencies, the infrared absorption and reflexion spectra are given and interpreted on the basis of a mechanical and a rigid-ion lattice-dynamical model. From the results a critical mode is predicted at the boundary of the Brillouin zone (L in R{\bar 3}m). Further, the Γ phonon becomes weak at the lower transition point. The normal coordinates of both modes give parallel and antiparallel deviations of Pb positions from the [111] axis of the rhombohedral R{\bar 3}m high-temperature phase.