Abstract
The paper describes Brillouin light scattering studies of longitudinal acoustic (LA)phonons in Cs5H3(SO4)4×x H2O (PCHS) crystals attemperatures from 100 K to 360 K.The acoustic response of the crystal at different frequencies is analysed in detail.It is shown that boththe velocity and damping of sound exhibit a strong dispersion caused by relaxationprocesses in the region of transformation into the glass-like phase (Tg≈260 K).A strong anisotropy in theacoustic response, attributable to the quasi-two dimensional (quasi-2D) structure of PCHS,is revealed.The LA phonon damping is calculated in the framework of a number of relaxationmodels.It is shown that, in the vicinity of Tg, anomalies in ultrasonic damping ofthe LA phononspropagating in the basal plane reflect the cooperative effect of freezing of acid protons.At thesame time, the anomaly in damping of the LA phonon propagating perpendicular to the basal planeis described in terms of the Debye model and is due to the interaction between protonson hydrogen bonds and LA phonons.This suggests that the proton glass state realized at T<Tg is ofquasi-2D nature.
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