Electrosynthesis and crystal structure of the new 15R hexagonal perovskite Ba5MnNa2V2O13

摘要

A new manganese oxide Ba5MnNa2V2O13 with an original structure closely related to the cubic perovskite has been prepared by electrosynthesis in molten NaOH. Its crystal structure has been refined from single crystal X-ray diffraction in the R (3) over barm space group, a = 5.8490(6) Angstrom, c = 36.856(5) Angstrom, Z = 3, R-1 = 4.72%, wR(2) = 10.56%. The crystal structure is a rhombohedral 15R polytype and exhibits a close packed structure built up from [BaO3-delta] (c) and [BaO2] (c') layers within a (ccc'cc)(3) stacking sequence. The resulting three-dimensional edifice is formed by Ba(Mn0.33Na0.67)O-3 cubic perovskite blocks separated by double sheets of V5+ O-4 tetrahedra pointing towards the central [BaO2]c' layer. In the perovskite blocks, 1/6 of oxygen deficient vacancies located on layers surrounding manganese involve a Mn(IV) valence, in square pyramids rather than octahedra. On the same layers along c, the barium atom split from a central (0, 0, z) position to close (x, -x, z) positions as a compensation of the oxygen deficiency. In this work, a review of the Mn-related perovskite materials found in the literature is reported showing the wide variety of materials adopting related structural polytypes. A building scheme from simple to more complex edifices is also presented by successive intercalation of [BaOn] (n = 1, 2) in order to visualize topological relationships between the number of possible hexagonal perovskite series members. (C) 2003 Elsevier Inc. All rights reserved.