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

Abstract

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 Eu_0.8Ca_0.2KGe₂O_6-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.