International Research Journal of Pure and Applied Chemistry

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.