Discovery of a Rare-Earth-Free Oxide-Ion Conductor Ca₃Ga₄O₉ by Screening through Bond Valence-Based Energy Calculations, Synthesis, and Characterization of Structural and Transport Properties

In this work, we have discovered Ca₃Ga₄O₉ as a rare-earth-free oxide-ion conductor by a combined technique of bond valence (BV)-based energy calculations, synthesis, and characterization of structural and transport properties. Here, the energy barriers for oxide-ion migration (E_b) of 217 Ga-containing oxides were calculated by the BV method to screen the candidate materials of oxide-ion conductors. We chose the orthorhombic calcium gallate Ca₃Ga₄O₉ as a candidate of oxide-ion conductors, because Ca₃Ga₄O₉ had a relatively low E_b. Ca₃Ga₄O₉ was synthesized by a solid-state-reaction method. Rietveld analyses of time-of-flight neutron and synchrotron X-ray powder diffraction data of Ca₃Ga₄O₉ indicated an orthorhombic Cmm2 layered crystal structure consisting of Ca₁₈ and (Ga₄O₉)₆ units where the (Ga₄O₉)₆ units form the two-dimensional (2D) corner-sharing GaO₄ tetrahedral network. The electromotive force measurements with an oxygen concentration cell showed that the transport numbers of the oxide ion were 0.69 at 1073 K and 0.84 at 973 K in Ca₃Ga₄O₉, which indicates that the major carrier of Ca₃Ga₄O₉ is the oxide ion. The oxide-ion conductivity was estimated to be 1.03(8) × 10^-5 S cm^-1 at 1073 K. The total electrical conductivity and impedance spectroscopy measurements of this Ca₃Ga₄O₉ sample indicated that the bulk conductivity was much higher than the grain-boundary conductivity and that the total conductivity was equivalent to the bulk conductivity. The bond valence-based energy landscape calculated using the refined crystal parameters of Ca₃Ga₄O₉ indicated 2D oxide-ion diffusion in the layered tetrahedral network [(Ga₄O₉)₆ unit]. It was found that the structural and transport properties of Ca₃Ga₄O₉ are similar to those of LaSrGa₃O₇ melilite.