Überstruktur des Strontiumvanadiumoxids, Sr_0,5V₂O₅: ihre Zwillingsbildung, Gruppoid-Symmetrie und Interpretation als OD-Struktur

The title compound has been considered to be isotypic with Ag 0.68 V 2 O 5 , a typical δ vanadium bronze crystallizing in space group C2/m. Through hydrothermal synthesis, however, crystals were obtained showing superlattice reflections. Triclinic, C1 (No. 2), a = 23.536(2) A, b = 7.3985(3) A, c = 8.801(1) A, α = 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, R w (F) = 0.024. Twinning about [100] or (001) was also observed. The structure includes V 2 O 5 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.