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

Abstract

We report the successful synthesis of three new Ruddlesden-Popper-type scandium oxychloride perovskites, SrScOCl, SrScOCl, and BaScOCl, by conventional solid-state reaction. Small single crystals of SrScOCl 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 ScOCl 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 ScO pyramid. These structural features in the oxychloride are different from those of octahedral ScOF coordinated with a partial O/F anion order at the apical sites in the oxyfluoride SrScOF. Rietveld refinements of the neutron powder diffraction data of SrScOCl ( I4/ mmm: a = 4.107982(5) Å, c = 23.58454(7) Å) and BaScOCl ( I4/ mmm: a = 4.206920(5) Å, c = 24.54386(6) Å) reveal the presence of pseudo ScO pyramids with the Cl anion being distant from the scandium cation, which is similar to the Sc-centered coordination geometry in SrScOCl with the exception that the ScO pyramids form double layers by sharing the apical oxygen. Density functional calculations on SrScOCl indicate the strong covalency of the Sc-O bonds but almost nonbonding interaction between Sc and Cl ions.

Layered Double Perovskites

“…For example, for a lot of the oxychlorides, the metalchloride bond lengths are often above 3Å;, which is much longer than the expected covalent bond length of 2.4Å; or below. Other researchers who have studied these compounds have taken contrasting views on how to view the B -X bonding; some have rationalized the compounds as LPD's with severely distorted metal-O/X octahedra, (121,55) while other have regarded them as B O5 square pyramids with counter X anions. (122,123,124) Specific to the oxyhalide field, previous research did not always take advantage of DFT calculations to support the structural characterization, but one recent work on a set of new scandium oxychlorides found that the metal halide association was essentially nonbonding, and the B cation environment is best described as square pyramidal.…”

“…(122,123,124) Specific to the oxyhalide field, previous research did not always take advantage of DFT calculations to support the structural characterization, but one recent work on a set of new scandium oxychlorides found that the metal halide association was essentially nonbonding, and the B cation environment is best described as square pyramidal. (125) There are, however, examples such as CsCaNb2O6F, (126) where the B -site metal-fluoride interaction is clearly bonding (bond length of 2.34Å). In general though, a defining feature of these anion ordered LDP structure type appears to be that they are strangely effective at inducing square pyramidal geometry in metals, even for those that are not known to commonly do so (Sc and In, in particular).…”

Ruddlesden–Popper Oxychlorides Ba3Y2O5Cl2, Sr3Sc2O5Cl2, and Sr2ScO3Cl: First Examples of Oxide-Ion-Conducting Oxychlorides

“…The crystal parameters of Ba 3 Y 2 O 5 Cl 2 , Sr 3 Sc 2 O 5 Cl 2 , and Sr 2 ScO 3 Cl were refined by the Rietveld method using the Z-code program . The crystal structures of Ba 3 Y 2 O 5 Cl 2 , Sr 3 Sc 2 O 5 Cl 2 , and Sr 2 ScO 3 Cl were refined using the crystal parameters in the literature , as initial parameters. X-ray fluorescence (XRF) spectra indicated the atomic ratios of Ba/Y/Cl = 3:2:2 for Ba 3 Y 2 O 5 Cl 2 , Sr/Sc/Cl = 3:2:2 for Sr 3 Sc 2 O 5 Cl 2 , and Sr/Sc/Cl = 2:1:1 for Sr 2 ScO 3 Cl, which agrees with the ideal ratios of Ba 3 Y 2 O 5 Cl 2 , Sr 3 Sc 2 O 5 Cl 2 , and Sr 2 ScO 3 Cl, respectively.…”

“…The bond-valence-based energy (BVE) − of a test oxide ion (O 2– ) in the crystal structures of Ba 3 Y 2 O 5 Cl 2 , Sr 3 Sc 2 O 5 Cl 2 , and Sr 2 ScO 3 Cl was obtained with the SoftBV program to study the oxide-ion migration paths. The BVE landscapes were calculated with a spatial resolution of approximately 0.1 Å using the crystal structures reported in our previous works. , The crystal structures and BVE landscapes were drawn with VESTA …”

“…Thermal ellipsoids are drawn at the 99% probability level. The crystal structures were depicted with the crystal data reported in our previous works. , Here A = Ba and Sr, B = Y and Sc, X = O and Cl, and n is the natural number n in A n +1 B n X 3 n +1 .…”