Synthesis, Crystal Structure, and Optical Properties of Layered Perovskite Scandium Oxychlorides: Sr₂ScO₃Cl, Sr₃Sc₂O₅Cl₂, and Ba₃Sc₂O₅Cl₂

We report the successful synthesis of three new Ruddlesden–​Popper-type scandium oxychloride perovskites, Sr₂Sc­O₃Cl, Sr₃Sc₂­O₅Cl₂, and Ba₃Sc₂­O₅Cl₂, by conventional solid-state reaction. Small single crystals of Sr₂Sc­O₃Cl were obtained by a self-flux method, and the crystal structure was determined to belong to the tetragonal P4/nmm space group (a = 4.08066(14) Å, c = 14.1115(8) Å) by X-ray diffraction analysis. The scandium center forms a ScO₅Cl octahedron with ordered apical oxygen and chlorine anions. The scandium cation, however, is shifted from the position of the octahedral center toward the apical oxygen anion, such that the coordination geometry of the Sc cation can be effectively viewed as an ScO₅ pyramid. These structural features in the oxychloride are different from those of octahedral ScO₅F coordinated with a partial O/F anion order at the apical sites in the oxyfluoride Sr₂Sc­O₃F. Rietveld refinements of the neutron powder diffraction data of Sr₃Sc₂­O₅Cl₂ (I4/mmm: a = 4.107982(5) Å, c = 23.58454(7) Å) and Ba₃Sc₂­O₅Cl₂ (I4/mmm: a = 4.206920(5) Å, c = 24.54386(6) Å) reveal the presence of pseudo ScO₅ pyramids with the Cl anion being distant from the scandium cation, which is similar to the Sc-centered coordination geometry in Sr₂Sc­O₃Cl with the exception that the ScO₅ pyramids form double layers by sharing the apical oxygen. Density functional calculations on Sr₂Sc­O₃Cl indicate the strong covalency of the Sc–O bonds but almost nonbonding interaction between Sc and Cl ions.