BRILLOUIN SPECTROSCOPY INVESTIGATION OF THE SUPERIONIC TRANSITION IN CSH2ASO4

A Brillouin scattering investigation in CsH2AsO4 has been performed ov er a temperature range 20 - 200 degrees C which includes the superioni c transition at T-S similar to 167 degrees C. At T-S, large discontinu ities are observed for elastic constants C-11, C-33 and C = (C-11 + C -12 + 2C(66)(E))/2 and, above T-S, a broadening of the corresponding l ines takes place. In the conducting phase above T-S the spectra reveal a domain structure. The results are discussed by comparison with comp ounds of CsHSeO4 type. It is shown that a Linear coupling between stra ins and mobile protons in the conducting phase can neither contribute to modification of elastic constants nor to broadening of lines. The e xperimental results reflect a strong general anharmonicity of the latt ice dynamics in the conducting phase.