Evidence for a quasi-two-dimensional proton glass state in Cs5H3(SO4)(4)center dot xH(2)O crystals

We describe damping of hypersonic and ultrasonic longitudinal acoustic (LA) phonons in crystals of Cs5H3(SO4)(4). xH(2)O (PCHS) between 100 and 360 K. The damping of LA phonons exhibits strong dispersion caused by relaxation processes in the region of transformation into the glasslike phase (T-g app roximate to 260 K). Near T-g the damping of ultrasonic phonons propagating in the basal plane reflects the cooperative freezing of acid protons. The d amping of LA phonons propagating perpendicular to the basal plane can be fi t by the Debye model and is due to the interaction between protons and LA p honons. This suggests that the proton glass state that is realized at T < T -g has a quasi-two-dimensional nature.