Crystal structure, conductivity, and phase stability of Cs3(H1.5PO4)2 under controlled humidity

Abstract

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(H1.5PO4)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(H1.5PO4)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(H1.5PO4)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.