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  • Author: Y. Kato x
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Abstract

The crystal structure of β-YbV4O8 has been determined using a twinned specimen and refined as a commensurate composite structure including V2O4 framework and Yb cations as substructures. The lattice dimensions for V2O4 are a 1 = 9.030(5) Å, b 1 = 10.72(2) Å, c 1 = 2.876(1) Å, α = 89.911(3)°; those for the Yb substructure are a 2 = a 1, b 2 = b 1C 1, c 2 = 2c 1. Both substructures have the space group P21/n11. The three-dimensional framework of V2O4 is isostructural with those of Fe2O4 in CaFe2O4 and Ti2O4 in NaTi2O4. The Yb cations occupy half of the [8]-coordinated sites in the tunnel of the framework running parallel to [001]. α- and β-YbV4O8 are polytypes in which the Yb cations are differently arranged while the V2O4 frameworks are essentially the same.

Abstract

SrV12O27 · 3 H2O, synthesized hydrothermally. Monoclinic P21/m; a = 11.684(2) Å, b = 11.126(2) Å, c = 9.052(2) Å, β = 110.94(2)°; Z = 2. R(F) = 0.049, RW(F) = 0.039 for 3756 observed reflections. The structure includes a new vanadate framework V4O9, similar to the V2O5 in vanadium bronzes β-AxV2O5. The V4O9 framework, forming a substructure with b′ = b/3, consists of 2 × 3 columns of edge-sharing VO6 octahedra and chains of edge-sharing VO5 trigonal bipyramids. The columns and the chains run parallel to [010] and are connected through common corners of VO6 and VO5. The columns neighbouring in [001] direction share two corners of octahedra per b′ period. The Sr atoms and H2O molecules are accommodated in the wide [010] tunnels characteristic of the V4O9 framework.

Abstract

The title compound has been considered to be isotypic with Ag0.68V2O5, a typical δ vanadium bronze crystallizing in space group C2/m. Through hydrothermal synthesis, however, crystals were obtained showing superlattice reflections. Triclinic, C[unk] (No. 2), a = 23.536(2) Å, b = 7.3985(3) Å, c = 8.801(1) Å, a = 90°, β = 88.28(1)°, γ = 90°, Z = 16. The structure was determined based on 2160 reflections of a specimen twinned about [010] | (010); R(F) = 0.032, Rw(F) = 0.024. Twinning about [100] or (001) was also observed. The structure includes V2O5 layers with Sr atoms sandwiched between them. The perfectly ordered arrangement of the Sr atoms leads to the superstructure. The V(V) and V(IV) positions can be distinguished. The structure has an idealized symmetry of a Brandt groupoid C2/m {t(1/4, 3/4, 0)}. An interpretation as OD structure is given to explain the mechanism of twinning.