Brillouin scattering observations of central peaks in ferroelastic LaP5O14

Three different kinds of narrow (linewidth < 150 MHz) central peaks were observed near the ferroelastic transition of very high optical quality samples of LaP5O14. The intensity of the first one is maximum some degrees below the transition and probably corresponds to domain wall reflections. The very intense second one is localized within 0.08 K below the transition. The third central peak can be observed in both sides of the transition and is induced by defects having the same symmetry than that of the soft acoustic mode.