Phase Behavior and Superprotonic Conductivity in the System (1–x)CsH2PO4 – xH3PO4: Discovery of Off-Stoichiometric α‑[Cs1–x H 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 [Cs1–x H 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 [Cs1–x H 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 [Cs1–x H 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.