Acoustic anomalies at phase transformation to quasi-2D proton glass state in Cs5H3(SO4)4.xH2O crystal

Abstract

The paper describes Brillouin light scattering studies of longitudinal acoustic (LA) phonons in Cs5H3(SO4)4×x H2O (PCHS) crystals at temperatures from 100 K to 360 K. The acoustic response of the crystal at different frequencies is analysed in detail. It is shown that both the velocity and damping of sound exhibit a strong dispersion caused by relaxation processes in the region of transformation into the glass-like phase (Tg≈260 K). A strong anisotropy in the acoustic 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 relaxation models. It is shown that, in the vicinity of Tg, anomalies in ultrasonic damping of the LA phonons propagating in the basal plane reflect the cooperative effect of freezing of acid protons. At the same time, the anomaly in damping of the LA phonon propagating perpendicular to the basal plane is described in terms of the Debye model and is due to the interaction between protons on hydrogen bonds and LA phonons. This suggests that the proton glass state realized at T<Tg is of quasi-2D nature.