VIBRATIONAL STUDY OF SUPERIONIC PHASE-TRANSITIONS IN THE MIXED-ALKALI ACID SULFATE CS0.5RB0.5HSO4

Raman spectra (10-1100 cm-1) of polycrystalline samples of Cs0.5Rb0.5HSO4 were studied at temperatures ranging from 300 to 500 K. An assignment of most of the observed band is proposed. The first-order phase transitions previously detected at 350 and 405 K were characterized: I --> 350 K II --> 405 K III These transitions are believed to be governed by librations of the SO42- ion and the S-OH stretching mode. The I --> II transition is characterized by the conversion of infinite (HSO4-)n chains to cyclic (HSO4-)2 dimers. The superionic II --> III transition corresponds to the weakening of SO ... H ... OS hydrogen bonds and to the melting of the proton sublattice to a quasi-liquid state where proton and SO42- ions contribute to the unusual high conductivity. The activation energy associated with the oscillation of the proton between two SO42- groups is E0 = 12.1 kJ mol-1. This was extracted from the plot of DELTAv1/2 vs. temperature for the vS-OH band at 875 cm-1. It is of the same order of magnitude as that estimated from conductivity measurements.