Syntheses, Crystal Structures, and Twinning of the Isotypic Rare‐Earth Metal Bromide Ortho‐Oxidomolybdates LaBrMoO4 and CeBrMoO4

Abstract

AbstractSolid‐state reactions of La2O3, LaBr3, and MoO3 in molar ratios of 1:1:3 lead to single crystals of LaBrMoO4 among a few by‐products, which emerge from redox reactions of Br– with hexavalent molybdenum. To obtain the analogous cerium compound, Ce2O3 was synthesized in situ by using CeO2 and Ce in a 3:1 molar ratio. The two rare‐earth metal bromide ortho‐oxidomolybdates(VI) REBrMoO4 (RE = La and Ce) crystallize isotypically in the monoclinic space group Pc (a = 982 – 977 pm, b ≈ 581 pm, c = 810 – 803 pm, β ≈ 90°, Z = 4). In the crystal structure, two crystallographically unique RE3+ cations are present, both with similar coordination environments of three plus one Br– and six O2– anions in the shape of distorted tetracapped trigonal prisms. The two distinguishable Br– anions both display coordination spheres of three plus one RE3+ cations each, building up distorted tetrahedra. These are fused together via four common edges to form anti‐PbO‐type 2∞{[BrRE$\rm^{e}_{4/4}$]2+} layers (e = edge‐connecting) parallel to the (100) plane. Two crystallographically different oxidomolybdate(VI) units are also found in the structure, which can be best described as strands of apically vertex‐shared [MoO5]4– trigonal bipyramids along [001] represented by the formula 1∞{[MoO$\rm^{e}_{4/4}$O$\rm^{e}_{4/4}$]2–} (v = vertex‐connecting, t = terminal). The aforementioned 2∞{[BrRE$\rm^{e}_{4/4}$]2+} layers and the 1∞ {[MoO$\rm^{e}_{4/4}$O$\rm^{e}_{4/4}$]2–} chains are alternately stacked along the a axis. The peculiarity of this structure is expressed by the position of the Mo6+ cations, which are not situated in the center of the bipyramids, but reside offset in a trigonal pyramid. Since both the (Mo1)6+ and the (Mo2)6+ cations are located in the same partial tetrahedron, relative to the triangular plane of the trigonal bipyramid, a structure description in a non‐centrosymmetric space group is immanent. Both title compounds display a twinned crystal structure with strong similarities to those of the chloride homologues REClMoO4 (RE = La – Pr) and LaClWO4.