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

The behavior of hypersonic longitudinal acoustic phonons at temperatures ra nging from 290 to 370 K in a crystal of Cs5H3(SO4)(4). nH(2)O was studied b y Brillouin light scattering. Anomalies in the temperature dependences of t he frequency shift and spectral width of the Brillouin components in the vi cinity of T approximate to 360 K were observed. We found a "point of isotro pization" near 360 K where C-11 approximate to C-33 approximate to 2.6 X 10 (10) N/m(2). We attribute it to an isostructural phase transition from P6(3 )/mmc double left right arrow P6(3)/mmc where the apparent acoustic symmetr y changes from hexagonal to cubic. Possible models for the phase transition have been discussed. (C) 2000 Published by Elsevier Science Ltd. All right s reserved.