Metal ion conducting ceramic materials are disclosed having characteristics of high ion conductivity for certain alkali and monovalent metal ions at low temperatures, high selectivity for the metal ions, good current efficiency and stability in water and corrosive media under static and electrochemical conditions. The metal ion conducting ceramic materials are fabricated to be deficient in the metal ion. One general formulation of the metal ion conducting ceramic materials is Me1+x+y−zMIIIyMIV2−ySixP3−xO12−z/2, wherein Me is Na+, Li+, K+, Rb+, Cs+, Ag+, or mixtures thereof, 2.0≦x≦2.4, 0.0≦y≦1.0, and 0.05≦z≦0.9, where MIII is Al3+, Ga3+, Cr3+, Sc3+, Fe3+, In3+, Yb3+, Y3+, or mixtures thereof and MIV is Ti4+, Zr4+, Hf4+, or mixtures thereof.