Precursors in lead phosphate-type ferroelastics: Diffuse X-ray scattering,group theory and modelling

A chain-adapted symmetry-mode analysis is presented for the symmetry change R(3) over bar m > C12/c1 in ferroelastic lead phosphate. The primary and t he secondary modes are derived for all relevant Wyckoff positions. The spli tting schemes of the orbits and the patterns of the displacive modes are co mpatible with experimental diffraction studies performed in lead phosphate type crystals. The appearance of diffuse monoclinic X-ray diffraction signa ls above the ferroelastic transition point of lead phosphate is modelled us ing simple real structure simulations.