Discovery of Oxide-Ion Conductors with a New Crystal Structure, BaSc_2–xA_xSi₃O_10–x/2 (A = Mg, Ca) by Screening Sc-Containing Oxides through the Bond-Valence Method and Experiments

We have discovered oxide-ion conductors with a new crystal structure by combining the bond-valence (BV) method and experiments. In the present work, the BV-based energy barrier Eb for oxide-ion migration was calculated for 123 kinds of Sc-containing oxides as a screening process. We found that the monoclinic BaGd2Si3O10-type barium scandium silicate BaSc2Si3O10 has a relatively low Eb, indicating that it is potentially a new oxide-ion conductor. BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A = Mg, Ca) were prepared by solid-state reactions. Rietveld analyses of X-ray powder diffraction data of these samples were successfully performed using the monoclinic P21/m BaGd2Si3O10-type structure. Lattice volume V of BaSc1.9Mg0.1Si3O9.95 was smaller than V of BaSc2Si3O10 due to the substitution of smaller sized Mg2+ for Sc3+, while the V of BaSc1.9Ca0.1Si3O9.95 was larger than V of BaSc2Si3O10 due to the substitution of larger sized Ca2+ for Sc3+, indicating the formation of solid solutions. Electrical conductivity and ultraviolet–visible (UV–vis) diffuse reflectance measurements indicated that the dominant carrier of BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A = Mg, Ca) was oxide ion. Thus, BaSc2Si3O10 and BaSc1.9A0.1Si3O9.95 (A = Mg, Ca) were found to be new structure-type oxide-ion conductors. These materials are the first examples of pure oxide-ion conductors containing Sc as an essential element. The oxide-ion conductivity of BaSc2Si3O10 was enhanced by doping Mg or Ca owing to the increase in the carrier (oxygen vacancy) concentration. The oxide-ion conductivities of BaSc1.9Mg0.1Si3O9.95 and BaSc1.9Ca0.1Si3O9.95 were approximately 19 times higher than that of BaSc2Si3O10 at 1000 °C. The BV-based energy landscape of BaSc2Si3O10 indicated two- or three-dimensional oxide-ion diffusion along the edges of Si3O10 groups and/or ScO6 octahedra.