Sr2(Mg1−xGax)Ge2O7+0.5x: Melilite-type oxygen ionic conductor associated with fivefold coordinated germanium and gallium

We demonstrate a new type of oxygen ionic conductor, Sr2(Mg1−xGax)Ge2O7+0.5x with a melilite-type structure. The oxygen-ion conductivity of Sr2Mg0.8Ga0.2Ge2O7.1 is measured as 10−4 S/cm at 800 °C using both 2-probe ac impedance and 4-probe dc measurements, and hardly depends on Po2 indicating that the main charge carriers are interstitial oxygen ions. Based on total energy calculations, it is suggested that a Ge4+ ion as well as Ga3+ contributes to the ionic conduction of interstitial oxygen through the coordination changes from fourfold to fivefold.