Proton conductivity and phase composition of mixed salts in the systems $M$H$_{2}$PO$_{4}$–CsHSO$_{4}$ ($M$ = Cs, K)

Phase transformations, electrical transport and thermal properties of the systems K$_{1-x}$Cs$_{x}$(H$_{2}$PO$_{4})_{1-x}$(HSO$_{4})_{x}$ ($x$ = 0.01–0.95) and Cs(H$_{2}$PO$_{4}$)$_{1-x}$ (HSO$_{4})_{x}$ ($x$ = 0.01–0.30) have been studied in detail. It has been shown that the mixed compounds Cs(H$_{2}$PO$_{4}$)$_{1-x}$(HSO$_{4})_{x}$ 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 $x\ge$ 0.15. The partial substitution of CsH$_{2}$PO$_{4}$ ions for the anions in HSO$_{4}^{-}$ at $x$ = 0.01–0.10 leads to the formation of Cs(H$_{2}$PO$_{4}$)$_{1-x}$(HSO$_{4})_{x}$ solid solutions isostructural with the CsH$_{2}$PO$_{4}$ $(P2_{1}/m)$ phase. For Cs(H$_{2}$PO$_{4}$)$_{1-x}$(HSO$_{4}$)$_{x}$ with $x$ = 0.15–0.30 at room temperature, there is a stabilization of the high-temperature cubic phase isostructural with the CsH$_{2}$PO$_{4}$ $(Pm\bar3m)$ phase existing in CsH$_{2}$PO$_{4}$ at temperatures above 230$^\circ$C. The stability of the $Pm\bar3m$ cubic phase at room temperature has been investigated using X-ray powder diffraction, $^{1}$H spectroscopy, and impedance spectroscopy. In the K$_{1-x}$Cs$_{x}$(H$_{2}$PO$_{4})_{1-x}$(HSO$_{4})_{x}$ 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 CsH$_{5}$(PO$_{4}$)$_{2}$ phase, which is stoichiometrically different from the initial salts, and the potassium-containing phase, which is isostructural with the superionic salt Cs$_{3}$(HSO$_{4}$)$_{2}$(H$_{2}$PO$_{4}$).