Phase Behavior and Superprotonic Conductivity in the System (1–x)CsH₂PO₄ – xH₃PO₄: Discovery of Off-Stoichiometric α‑[Cs_1–xH_x]H₂PO₄

CsH₂PO₄ has garnered interest as a proton-conducting electrolyte due to its exceptional conductivity at intermediate temperatures (228–300 °C) at which it adopts a cubic structure with a high degree of disorder. Here, through a study of mixtures of CsH₂PO₄ (CDP) and CsH₅(PO₄)₂, the cubic phase was discovered to form solid solutions of composition [Cs_1–xH_x]­H₂PO₄, with x extending to at least 2/9. A phase diagram of the composition space (1–x)­CsH₂PO₄ – xH₃PO₄, 0 ≤ x ≤ 2/9 was developed through thermal analysis, high-temperature in situ X-ray diffraction experiments, and variable-temperature NMR spectroscopy. At temperatures above about 90 °C, monoclinic, stoichiometric CDP exists in equilibrium with Cs₇(H₄PO₄)­(H₂PO₄)₈. These two phases displayed eutectoid behavior, with a eutectoid reaction temperature and composition of 155 °C and x = 0.18, respectively, to form cubic [Cs_1–xH_x]­H₂PO₄. The structural studies revealed, rather remarkably, that the cubic phase accommodates vacancies on the cation site that are charge-balanced by excess protons, where the latter are chemically associated with phosphate groups. The conductivities of cubic phases of various compositions, measured by impedance spectroscopy, are comparable to that of CDP. The excellent proton conductivities of off-stoichiometric, cubic [Cs_1–xH_x]­H₂PO₄ at temperatures well below the superprotonic transition of stoichiometric CDP present the opportunity to extend the low-temperature operating limit of CDP-based devices. More generally, the off-stoichiometric phase behavior demonstrated here introduces a new approach for the modification of superprotonic solid acid compounds.