Crystal structure, conductivity, and phase stability of Cs₃(H_1.5PO₄)₂ under controlled humidity

Upon heating, a number of solid acids of structure type M₃H(XO₄)₂ (M = NH₄, K, Rb, Cs; X = S, Se) transform from a low conductivity monoclinic phase into a high conductivity (superprotonic) phase. Here, a new member of this material class, Cs₃(H_1.5PO₄)₂, is studied using a combination of single crystal X-ray diffraction, in-situ powder diffraction, thermogravimetric analysis, and A.C. impedance spectroscopy. The room temperature crystal structure of Cs₃(H_1.5PO₄)₂ was solved in the C2/m space group and found to be essentially isostructural to Cs₃H(SeO₄)₂. On heating, no transition to a superprotonic phase was observed. Instead, Cs₃(H_1.5PO₄)₂ exsolves a small quantity of a CsH₂PO₄-like cubic phase at an onset temperature of ~192 °C prior to decomposition. The absence of a superprotonic phase transition is discussed in the context of other M₃H(XO₄)₂ solid acids and the crystal-chemical features that define this class of materials.