Effect of cation substitution in Cs$_{1-2x}$Ba$_{x}$H$_{2}$PO$_{4}$ on structural properties and proton conductivity

We synthesized compounds with partial substitution of Cs$^+$ cations in CsH$_{2}$PO$_{4}$ by Ba$^{2+}$ cations. The structural, electron transport and thermodynamic properties of Cs$_{1-2x}$Ba$_{x}$H$_{2}$PO$_{4}$ ($x$ = 0–0.15) were studied for the first time with the help of a set of physicochemical methods: infrared and impedance spectroscopy, X-ray diffraction and synchronous thermal analysis. The proton conductivity of Cs$_{1-2x}$Ba$_{x}$H$_{2}$PO$_{4}$ at 50–230$^\circ$C was investigated in detail by impedance measurements. The formation of solid substitution solutions isostructural with CsH$_{2}$PO$_{4}$ ($P$2$_{1}/m$) is observed in the range of substitution degrees of $x$ = 0–0.1, with a slight decrease in the unit cell parameters and some salt amorphization. The conductivity of disordered Cs$_{1-2x}$Ba$_{x}$H$_{2}$PO$_{4}$ in the low-temperature region increases by two orders of magnitude at $x$ = 0.02 and increases with an increasing fraction of barium cations by three or four orders of magnitude at $x$ = 0.05–0.1; the superionic phase transition practically disappears. At $x$ = 0.15, heterophase systems based on salts are formed, showing high conductivity and a further decrease in the activation energy of conductivity to 0.63 eV. The conductivity of the high-temperature phase of Cs$_{1-2x}$Ba$_{x}$H$_{2}$PO$_{4}$ does not change with increasing fraction of the substituent.