Ge-Containing Oxide-Ion Conductors with CaEu 2 Ge 3 O 10 -Type Structure Discovered by the Bond-Valence Method and Experiments.

In the present work, we have discovered the first example of a CaEu 2 Ge 3 O 10 -type oxide-ion conductor, Ca 1.05 Sm 1.95 Ge 3 O 9.975 . The CaEu 2 Ge 3 O 10 -type structure was selected by screening 624 Ge-containing materials by the bond-valence-based-energy calculations. CaEu 2 Ge 3 O 10 -type CaEu 2 Ge 3 O 10 , CaGd 2 Ge 3 O 10 , and a new material CaSm 2 Ge 3 O 10 were synthesized. CaSm 2 Ge 3 O 10 showed the highest electrical conductivity among these three materials. Ca 1+ x Sm 2- x Ge 3 O 10- x /2 ( x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca 1.05 Sm 1.95 Ge 3 O 9.975 exhibited the highest conductivity of 1.2 × 10 -5 S cm -1 at 1373 K. Oxygen transport numbers in Ca 1.05 Sm 1.95 Ge 3 O 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, CaEu 2 Ge 3 O 10 -type Ca 1.05 Sm 1.95 Ge 3 O 9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm 2 Ge 3 O 10 and Ca 1.05 Sm 1.95 Ge 3 O 9.975 were successfully analyzed by the CaEu 2 Ge 3 O 10 -type structure (space group P 2 1 / c ) using the single-crystal X-ray diffraction data. The bond-valence-based-energy calculation for the refined crystal structure of Ca 1.05 Sm 1.95 Ge 3 O 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 1.05 Sm 1.95 Ge 3 O 9.975 .