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

Abstract

We\nreport the successful synthesis of three new Ruddlesden–​Popper-type\nscandium oxychloride perovskites, Sr₂Sc­O₃Cl, Sr₃Sc₂­O₅Cl₂, and Ba₃Sc₂­O₅Cl₂, by conventional solid-state reaction. Small single crystals of\nSr₂Sc­O₃Cl were obtained by a self-flux\nmethod, and the crystal structure was determined to belong to the\ntetragonal <i>P</i>4/<i>nmm</i> space group (<i>a</i> = 4.08066(14) Å, <i>c</i> = 14.1115(8)\nÅ) by X-ray diffraction analysis. The scandium center forms a\nScO₅Cl octahedron with ordered apical oxygen and chlorine\nanions. The scandium cation, however, is shifted from the position\nof the octahedral center toward the apical oxygen anion, such that\nthe coordination geometry of the Sc cation can be effectively viewed\nas an ScO₅ pyramid. These structural features in the oxychloride\nare different from those of octahedral ScO₅F coordinated\nwith a partial O/F anion order at the apical sites in the oxyfluoride\nSr₂Sc­O₃F. Rietveld refinements of the\nneutron powder diffraction data of Sr₃Sc₂­O₅Cl₂ (<i>I</i>4/<i>mmm</i>: <i>a</i> = 4.107982(5) Å, <i>c</i> = 23.58454(7)\nÅ) and Ba₃Sc₂­O₅Cl₂ (<i>I</i>4/<i>mmm</i>: <i>a</i> = 4.206920(5) Å, <i>c</i> = 24.54386(6) Å) reveal\nthe presence of pseudo ScO₅ pyramids with the Cl anion\nbeing distant from the scandium cation, which is similar to the Sc-centered\ncoordination geometry in Sr₂Sc­O₃Cl with\nthe exception that the ScO₅ pyramids form double layers\nby sharing the apical oxygen. Density functional calculations on Sr₂Sc­O₃Cl indicate the strong covalency of the\nSc–O bonds but almost nonbonding interaction between Sc and\nCl ions.