Transition de phase ferroélastique du phosphate de plomb. Etude par diffusion inélastique des neutrons

2014 An inelastic neutron scattering study of the ferroelastic phase transition in lead phosphate has revealed in the high temperature phase the existence of a zone boundary soft mode whose character is compatible with the structure change at the transition. The analysis of the observed spectral line shape shows that this soft mode is overdamped between Tc and Tc + 100 K. The quasiharmonic frequency is found to increase faster, as a function of temperature, then predicted by mean field theory. In the low temperature phase the zone center soft mode observed by Raman scattering has been identified ; in addition, and due to the monodomain nature of the sample, the corresponding zone boundary soft mode behaviour has been observed separately. The investigation of the acoustic mode dispersion in the principal directions of the crystal yields a good agreement with the Brillouin measurement for the corresponding sound velocities. LE JOURNAL DE PHYSIQUE TOME 40, DÉCEMBRE 1979,