On the Phase Transitions of Cesium Hydrogen Sulfate (CsHSO4)

V. Varma, N. Rangavittal, and C. N. R. Rao (J. Solid State Chem. 106, 164 (1993)) have concluded from infrared and Raman spectroscopic studies of CsHSO4 that reversible phase transitions occur near both 340 and 380 K. However, both transitions are "irreversible"; i.e., they do not occur spontaneously when the temperature is decreased. Actually, three different monoclinic phases can exist "indefinitely" at room temperature. The decisive factor is the water vapor pressure above the salt, and changes in water vapor pressure can trigger phase transitions in the bulk of the salt. Similar triggering can be caused by grinding, pellet pressing, etc. An analysis of the results reported by Varma et al. leads to the conclusion that their phases "IV" and "III" were not pure, but instead were mixtures of two or three monoclinic phases.