Thermal Properties and Structures of CsHSO4 and CsDSO4 Crystals

Differential scanning calorimetry, thermogravimetric-differential thermal analysis, and X-ray diffraction measurements were performed on cesium hydrogen sulfate (CsHSO4) and deuterated CsDSO4 crystals. The proton and deuterated compounds were confirmed to exhibit the superionic phase transition at 415.9 and 413.4 K, respectively. The II-III transition for the proton compound was observed in the temperature range of about 330-400 K. The thermal decomposition and dehydration reactions of both compounds began at around 460 K. The decomposition continued up to around 1050 K, and the dehydration ended at around 720 K. The weight losses in the temperature ranges of 460-720 K and 720-1050 K were caused by the evaporation of H(D)2O and SO3, respectively. The space group symmetries and structural parameters, in phase III (monoclinic, P21/n) for CsHSO4 and in phase II (monoclinic, P21/c) for CsHSO4 and CsDSO4, were determined at room temperature. The expansion of O-H-O hydrogen bond caused by the substitution of deuterium for hydrogen was observed to be 0.015(4) Å. The geometric isotope effect on hydrogen-bond structure upon deuteration was realized in the CsHSO4 crystal. The difference in morphology between as-grown CsHSO4 and CsDSO4 crystals was suggested to be caused by the large expansion of the O-H-O hydrogen bond upon deuteration on crystallization in D2O aqueous solution.