A new type of polycrystalline potassium ion conducting solid electrolyte doped with potassium nitrite as the starting material, was developed. Since cubic rare earth oxides hold reasonably enough interstitial space for bulky K+ ion migration in the crystal lattice, an extraordinary high K+ ion conductivity was successfully achieved by forming a polycrystalline (1 - x)R2O3-xKNO(3) solid solution, which was realized by doping KNO2 as the KNO3 state into the interstitial site of cubic rare earth oxide crystal lattice. The potassium ion conductivity of the (1 - x)R2O3-xKNO(3) (R: rare earths) solid solution linearly increased with expanding the R2O3 crystal lattice and the highest K+ ion conductivity was obtained for the 0.653Gd(2)O(3)-0.347KNO(3) solid solution, which is three orders of magnitude higher than that of a well-known polycrystalline K+ ion conducting K2SO4 solid and the value exceeds that in the ab conducting plane of a K+-beta"-Al2O3 single crystal. (c) 2005 Springer Science + Business Media, Inc.