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

The elastic properties of Cs3H(SeO4)(2) are investigated by Brillouin spect roscopy in the temperature range 20-220 degrees C covering the two transiti ons III --> II and II --> I occurring at similar to 95 degrees C and simila r to 187 degrees C; respectively. Phase I is known to be a protonic conduct ive one. Discontinuities of elastic constants are generally observed at bot h transitions, implying first orderness. In phases II and I, a slight broad ening of the Brillouin lines is detected. The results are discussed in comp arison with compounds of the families XHSeO4 (X = NH4, Rb and Cs) and CsH2B O4 (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.