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

Phase transformations, electrical transport and thermal properties of the systems K1‒xCsx(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 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Pm\overline 3 m$$\end{document}) phase existing in CsH2PO4 at temperatures above 230°C. The stability of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Pm\overline 3 m$$\end{document} cubic phase at room temperature has been investigated using X-ray powder diffraction, 1H NMR spectroscopy, and impedance spectroscopy. In the K1–xCsx(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).