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

Sheel Sanghvi, Sossina M. Haile*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


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.

Original languageEnglish (US)
Article number115291
JournalSolid State Ionics
StatePublished - Jun 2020


  • Cs(HPO)
  • CsH(SeO)
  • MH(XO)
  • Proton conductor
  • Solid acid
  • Superprotonic phase transition

ASJC Scopus subject areas

  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics


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