A Study of Superionic CsHSO4 and Cs1-xLixHSO4 by Vibrational Spectroscopy and X-Ray Diffraction

Infrared and Raman spectroscopic studies of CsHSO4 suggest the occurrence of the following phase transitions:IV ⇆ 340 K III ⇆ 380 K II ⇆ 417 K I. While the ordered phase, IV, has a monoclinic structure, the high-temperature superionic phase, I, has a tetragonal structure involving the free rotation of HSO- 4. Phase II is close to being orthorhombic with a small monoclinic distortion. The transition to the superionic phase, I, is accompanied by the appearance of broad, structureless bands in the infrared and Raman spectra, as well as changes in the O-H stretching region due to decrease in hydrogen bonding. Accompanying the III-II transition, we observe the disappearance of the O ··· O stretching and δ(S-OH) bands and a decrease in the HSO- 4 libration frequency. Across the phase transitions, the separation between the v(S-O) and v(S-OH) bands shows stepwise changes. The high-temperature superionic phase essentially has sulfate-like species with a relatively short S-O bond, in contrast to the low-temperature phase, which has long and short S-O distances. Substitution of Cs by Li in CsHSO4 has an effect similar to that of increasing the temperature. The vibrational spectrum of Cs0.7Li0.3HSO4 corresponds to a state of disorder between those of phases II and I.