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

Abstract

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(H1.5PO4)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 reacts with Cs3(H1.5PO4)2 to form α″-CDP, a cubic variant of superprotonic CsH2PO4, in which Cs:P exceeds 1:1. Thus, cubic CsH2PO4, which crystallizes in the CsCl structure-type, can support a large excess of Cs. Observation of a lattice parameter that decreases with increasing Cs content indicates that the chemistry is accommodated via the presence of phosphate vacancies rather than Cs interstitials. This interpretation of the chemistry is supported by Rietveld structure refinement. Charge balance is presumed to be maintained via a concomitant decrease in the average number of protons per phosphate group. Thus, the stoichiometry of α″-CDP, the notation adopted here to denote the phosphate deficiency in the cubic phase, is described as CsH2-3y(PO4)1-y. The conductivity of the α″-CDP materials (x = 0.05, 0.10, 0.17 and 0.18) approaches that of stoichiometric, superprotonic CDP, while providing access to a substantially wider temperature range of superprotonic transport.