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 pha se transitions were previously detected at 333 and 408 K. These transi tions are believed to be governed by librations of the AO(4)(2-) ion a nd the A-OH (A = S, Se) stretching mode. The transition at 333 K is ch aracterized by the conversion of infinite (HAO(4)(-))(n) with (A = S, Se) chains to cyclic (HAO(4)(-))(2) dimers. The transition at 408 K co rresponds to a superprotonic phase transition (SPT) where proton and A O(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 s ame order of magnitude as that estimated from conductivity.