Brillouin light scattering anomalies and new phase transition in Cs5H3(SO4)4 crystals

The behavior of hypersonic longitudinal acoustic phonons at temperatures ranging from 290 to 370 K in a crystal of Cs5H3(SO4)4·nH2O was studied by Brillouin light scattering. Anomalies in the temperature dependences of the frequency shift and spectral width of the Brillouin components in the vicinity of T≈360 K were observed. We found a “point of isotropization” near 360 K where C11≈C33≈2.6×1010 N/m2. We attribute it to an isostructural phase transition from P63/mmc⇔P63/mmc where the apparent acoustic symmetry changes from hexagonal to cubic. Possible models for the phase transition have been discussed.