Rubidium-cation conductivity of Rb$_{3-2x}$Pb$_{x}$PO$_{4}$ solid solutions

New Rb$_{3}$PO$_{4}$-based ceramic materials with high rubidium-cation conductivity in the Rb$_{3-2x}$Pb$_{x}$PO$_{4}$ system have been synthesized and studied. Introduction of Pb$^{2+}$ cations leads to a sharp increase in the conductivity of rubidium orthophosphate due to formation of cation vacancies and, at temperatures 350–550$^\circ$C, also due to the stabilization of high-temperature cubic modification Rb$_{3}$PO$_{4}$. At high temperatures, the electrolytes prepared have very high ion conductivity higher than 10$^{-1}$ S cm$^{-1}$ at 700$^\circ$C, which is higher than the values previously obtained in similar systems with additions of tin and cadmium ions. The factors influencing the transport properties of the materials under study are discussed.