Abstract
The existence of static and dynamic deformation in the intermediate phase between the high and low temperature phases (para- and ferroelastic) has been examined by high resolution neutron scattering, X-ray diffusion and Raman spectroscopy. The behaviour of the order parameter was investigated through the measurement of optical birefringence under shear stress. It was found that the structural deformation of the intermediate phase, as compared with the high temperature structure, is similar to that of the ferroelastic phase but with three nearly equivalent binary axes of the monoclinic unit cell. The spontaneous thermal switching between these three axes as a dynamical effect (flip-mode) is observed in Pb3(PO4)2. In Pb3(P1-xAs xO4)2 static deformations occur at temperatures just above the ferroelastic transition point and are superposed by flipping motion at higher temperatures