Vibrational study of superionic phase transitions in the mixed alkali acid sulphate Cs0.5Rb0.5HSO4

Abstract

AbstractRaman 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: These transitions are believed to be governed by librarians of the SO 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 SO ions contribute to the unusual high conductivity. The activation energy associated with the oscillation of the proton between two SO groups is E0 = 12.1 kJ mol−1. This was extracted from the plot of Δν1/2 vs. temperature for the νSOH band at 875 cm−1. It is of the same order of magnitude as that estimated from conductivity measurements.