Rubidium-cation conductivity in Rb$_{3-2x}$M$_x$PO$_4$ (M = Zn, Cd) solid solutions

New ceramic materials with a high rubidium-cation conductivity in Rb$_{3-2x}$M$_x$PO$_4$ (M = Zn, Cd) systems have been synthesized and studied. It has been found that the introduction of Zn$^{2+}$ and Cd$^{2+}$ cations leads to a sharp increase in the electrical conductivity of rubidium orthophosphate due to the formation of cation vacancies and the stabilization of the high-temperature cubic modification of Rb$_3$PO$_4$. The maximum conductivity has been observed in compositions with $x$ = 0.15–0.30 and has the values of 10$^{-1}$ S cm$^{-1}$ at 700$^\circ$C and (2.0–2.5) $\cdot$ 10$^{-2}$ S $\cdot$ cm$^{-1}$ at 400$^\circ$C. The factors influencing the transport properties of the materials under study have been discussed.