Ge-Containing Oxide-Ion Conductors with CaEu₂Ge₃O₁₀-Type Structure Discovered by the Bond-Valence Method and...

In the present work, we have discovered the first example of a CaEu₂Ge₃O₁₀-type oxide-ion conductor, Ca_1.05Sm_1.95Ge₃O_9.975. The CaEu₂Ge₃O₁₀-type structure was selected by screening 624 Ge-containing materials by the bond-valence-based-energy calculations. CaEu₂Ge₃O₁₀-type CaEu₂Ge₃O₁₀, CaGd₂Ge₃O₁₀, and a new material CaSm₂Ge₃O₁₀ were synthesized. CaSm₂Ge₃O₁₀ showed the highest electrical conductivity among these three materials. Ca₁₊Sm_2-Ge₃O_10- (x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca_1.05Sm_1.95Ge₃O_9.975 exhibited the highest conductivity of 1.2 × 10^-5 S cm^-1 at 1373 K. Oxygen transport numbers in Ca_1.05Sm_1.95Ge₃O_9.975 were determined to be 0.64(5) at 1073 K and 0.65(8) at 1123 K, which indicates that the major carrier is the oxide ion. Therefore, CaEu₂Ge₃O₁₀-type Ca_1.05Sm_1.95Ge₃O_9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm₂Ge₃O₁₀ and Ca_1.05Sm_1.95Ge₃O_9.975 were successfully analyzed by the CaEu₂Ge₃O₁₀-type structure (space group P2₁/c) using the single-crystal X-ray diffraction data. The bond-valence-based-energy calculation for the refined crystal structure of Ca_1.05Sm_1.95Ge₃O_9.975 suggested that oxide ions migrate along the [2 0 1], [0 1 0], and [12.88 6.43 1] directions with energy barriers of 0.88, 0.92, and 1.1 eV, respectively, which indicates three-dimensional oxide-ion diffusion in Ca_1.05Sm_1.95Ge₃O_9.975.