A new high copper ion conductivity solid electrolyte has been found in the system. This compound has a copper ion conductivity of 0.34 (Ω‐cm)−1 at 25°C, and activation energies for conduction of 7.0 kJ/mole between 130°C and −25°C, and 19 kJ/mole between −25°C and −140°C, respectively. Powder x‐ray diffraction analysis indicates that the crystal structure of is simple cubic with .