Vibrational study of the superionic-protonic phase transition of CsH(SO4)0.76(SeO4)0.24

Raman spectra (10-1300 cm(-1)) of polycrystalline samples of CsH(SO4)(0.76) (SeO4)(0.24) 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 AO(4)(2-) ion and the A-OH (A = S, Se) stretching mode. The transition at 333 K is characterized by the conversion of infinite (HAO(4)(-))(n) with (A = S, Se) chains to cyclic (HAO(4)(-))(2) dimers. The transition at 408 K corresponds to a superprotonic phase transition (SPT) where proton and AO(4)(2-) (A = S, Se) ions contribute to the unusual high conductivity. The activation energy was determined from the plot of Delta theta(1/2) as a function of temperature for the nu(se-OH) bands. It is of the same order of magnitude as that estimated from conductivity.