Phase behavior, crystal structure, and superprotonic conductivity of Cs[(H2PO4)1−2y(HPO4)y]: phosphate deficient analogs to cubic CsH2PO4 in the (1 − x)CsH2PO4–xCs2HPO4 system†

A systematic study of the (1 − x)CsH₂PO₄–xCs₂HPO₄ system has been carried out to explore the possibility of modifying the phase behavior of CsH₂PO₄ in the high temperature, superprotonic regime. Materials with x from 0 to 0.20 were characterized by in situ X-ray powder diffraction, simultaneous thermal analysis, and electrical impedance spectroscopy under a range of steam partial pressures. From these data, the phase diagram between CsH₂PO₄ (x = 0) and Cs₃(H_1.5PO₄)₂ (x = 0.5) was determined. The system displays eutectoid behavior, with an invariant point defined by a temperature of 192.0 ± 1.4 °C and a composition of x = 0.17 ± 0.01. At the eutectoid temperature, monoclinic CsH₂PO₄ combines with Cs₃(H_1.5PO₄)₂ to form α′′-CDP, a cubic variant of superprotonic CsH₂PO₄, in which Cs : P exceeds 1 : 1. This surprising result implies that cubic CsH₂PO₄, which crystallizes in the CsCl structure-type, can support a large excess of Cs. Rietveld structure refinement, along with a lattice parameter that decreases with increasing Cs content, reveals that the chemistry is accommodated via the presence of phosphate vacancies rather than Cs interstitials. Charge balance is presumed to be maintained via a concomitant decrease in the average number of protons per phosphate group. Accordingly, the stoichiometry of α′′-CDP is described as CsH_2−3y(PO₄)_1−y, and the phosphate vacancy concentration can be at least as high as 17% (x = 0.20). The conductivity of the α′′-CDP materials is comparable to that of stoichiometric, superprotonic CDP, while providing access to a substantially wider temperature range of superprotonic transport. This study reveals the potential for creating advanced proton conductors using cation:anion off-stoichiometry as a new design principle.