Hypersonic wave anomalies in Cs3H(SeO4)2 at the transitions above room temperature

The elastic properties of Cs3H(SeO4)(2) are investigated by Brillouin spectroscopy in the temperature range 20-220 degrees C covering the two transitions III --> II and II --> I occurring at similar to 95 degrees C and similar to 187 degrees C; respectively. Phase I is known to be a protonic conductive one. Discontinuities of elastic constants are generally observed at both transitions, implying first orderness. In phases II and I, a slight broadening of the Brillouin lines is detected. The results are discussed in comparison with compounds of the families XHSeO4 (X = NH4, Rb and Cs) and CsH2BO4 (B = As and P) which also undergo a transition to a superionic phase. In the conductive phase, it appears that the lattice anharmonicity is weaker in Cs3H(SeO4)(2) than in these other compounds.