Sn. Gvasaliya et al., Acoustic anomalies at phase transformation to quasi-2D proton glass state in Cs5H3(SO4)(4) x xH(2)O crystal, J PHYS-COND, 13(15), 2001, pp. 3677-3688
The paper describes Brillouin light scattering studies of longitudinal acou
stic (LA) phonons in Cs5H3(SO4)(4) X xH(2)O (PCHS) crystals at temperatures
from 100 K to 360 K. The acoustic response of the cn stal at different fre
quencies is analysed in detail. It is shown that both the velocity and damp
ing of sound exhibit a strong dispersion caused by relaxation processes in
the region of transformation into the glass-like phase (T-g approximate to
260 K). A strong anisotropy in the acoustic response, attributable to the q
uasi-two dimensional (quasi-2D) structure of PCHS, is revealed. The LA phon
on damping is calculated in the framework of a number of relaxation models.
It is shown that, in the vicinity of T-g, anomalies in ultrasonic damping
of the LA phonons propagating in the basal plane reflect the cooperative ef
fect 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 o
n hydrogen bonds and LA phonons. This suggests that the proton glass state
realized at T < T-g is of quasi-2D nature.