Vibrational study of the superionic - protonic phase transition of

Raman spectra ) of polycrystalline samples of were studied from 300 to 500 K. The first-order phase transitions were previously detected at 333 and 408 K. These transitions are believed to be governed by librations of the ion and the A - OH (A = S, Se) stretching mode. The transition at 333 K is characterized by the conversion of infinite with (A = S, Se) chains to cyclic dimers. The transition at 408 K corresponds to a superprotonic phase transition (SPT) where proton and (A = S, Se) ions contribute to the unusual high conductivity. The activation energy was determined from the plot of as a function of temperature for the bands. It is of the same order of magnitude as that estimated from conductivity.