Synthesis and Ion-Transport Properties of EuKGe2O6-, Ca3Fe2Ge3O12-, and BaCu2Ge2O7-Type Oxide-Ion Conductors

EuKGe2O6-, Ca3Fe2Ge3O12-, and BaCu2Ge2O7-type germanates are synthesized by a conventional solid-state method and characterized to reveal their oxide-ion-conducting properties. Materials of the EuKGe2O6 group exhibit oxide-ion conductivity (e.g., 4.6 × 10-3 S/cm at 973 K for Eu0.8Ca0.2KGe2O6-δ) and transport numbers above 96%, whereas materials of the Ca3Fe2Ge3O12 and BaCu2Ge2O7 groups exhibit mixed electron-/oxide-ion conduction. Conduction involves oxide-ion vacancies in the EuKGe2O6 group, interstitial oxide ions in the Ca3Fe2Ge3O12 group, and both oxide-ion vacancies and interstitial oxide ions in the BaCu2Ge2O7 group. The doping-induced formation of impurity phases decreases the amount of oxide-ion carriers relative to the expected values.