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 Me 1+x+y-z M III y M IV 2-y Si x P 3-x O 12-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 M III is Al 3+ , Ga 3+ , Cr 3+ , Sc 3+ , Fe 3+ , In 3+ , Yb 3+ , Y 3+ , or mixtures thereof and M IV is Ti 4+ , Zr 4+ , Hf 4+ , or mixtures thereof.