Proton conductivity and phase composition of mixed salts in the systems MH2PO4–CsHSO4 (M = Cs, K)

Abstract

Phase transformations, electrical transport and thermal properties of the systems K_1‒xCs_x(H₂PO₄)_1–x(HSO₄)_x (x = 0.01–0.95) and Cs(H₂PO₄)_1–x(HSO₄)_x (x = 0.01–0.30) have been studied in detail. It has been shown that the mixed compounds Cs(H₂PO₄)_1–x(HSO₄)_x are characterized by an increase in the low-temperature electrical conductivity by one to five orders of magnitude depending on the composition, as well as by the disappearance of the superionic phase transition at x ≥ 0.15. The partial substitution of HSO₄^- ions for the anions in CsH₂PO₄ at x = 0.01–0.10 leads to the formation of Cs(H₂PO₄)_1‒x(HSO₄)_x solid solutions isostructural with the CsH₂PO₄ (P2₁/m) phase. For Cs(H₂PO₄)_1–x(HSO₄)_x with x = 0.15–0.30 at room temperature, there is a stabilization of the high-temperature cubic phase isostructural with the CsH₂PO₄ (\(Pm\overline 3 m\)) phase existing in CsH₂PO₄ at temperatures above 230°C. The stability of the \(Pm\overline 3 m\) cubic phase at room temperature has been investigated using X-ray powder diffraction, ¹H NMR spectroscopy, and impedance spectroscopy. In the K_1–xCs_x(H₂PO₄)_1–x(HSO₄) system, there are two regions of compositions with x = 0.05–0.50 and 0.60–0.95, where the proton conductivity and thermal properties are determined respectively by the formation of the CsH₅(PO₄)₂ phase, which is stoichiometrically different from the initial salts, and the potassium-containing phase, which is isostructural with the superionic salt Cs₃(HSO₄)₂(H₂PO₄).