Dehydration of CsH₂PO₄ at temperatures higher than 260 °C and the ionic conductivity of liquid product

The dehydration of the superprotonic conductor CsH₂PO₄ has been carefully examined above 260 °C under highly humidified conditions. A.C. impedance spectroscopy on heating revealed that the dehydration of CsH₂PO₄ was accompanied by an increase in conductivity, whereas at temperatures below 260 °C, CsH₂PO₄ dehydrates to solid CsPO₃ with decrease of conductivity. Based on a combination of thermogravimetric analyses, it was clarified that this phenomena was caused by the dehydration of CsH₂PO₄ to form a liquid phase as follows: CsH₂PO₄(s) → CsH_{2(1 - x)}PO_{4 - x}(l) + xH₂O(g). It was also determined that the liquid phase dehydrates to solid CsPO₃ at higher temperatures and the stable region of CsH₂PO₄, dehydration liquid, and CsPO₃ were established in the diagram of temperature and partial pressure of water. In addition, the electrical property of the dehydration liquid was measured by a.c. impedance spectroscopy as well as the d.c. polarization method. The liquid phase exhibited high ionic conductivity, 5(± 4) × 10^{- 2} S cm^{- 1}, at 283.5 °C and p_H2O = 0.3 atm.