β-Nb₉VO₂₅

The title compound, nona­niobium vanadium penta­cosa­oxide, was prepared by a solid-state reaction at 1198 K. It is isotypic with Nb₉AsO₂₅, Nb₉PO₂₅ and Ta₉VO₂₅. The structure consists of NbO₆ octa­hedra (one with 4/m.. and two with m.. symmetry) and VO₄ tetra­hedra (-4.. symmetry) sharing corners and edges to form a three-dimensional framework. This framework can be considered as a junction between ribbons made up from NbO₆ octa­hedra and chains of NbO₆ octa­hedra and chains of VO₄ tetra­hedra. The V site shows half-occupancy, hence one half of the VO₄ tetra­hedra is unoccupied. The structural differences with α-Nb₉VO₂₅, VOSO₄, SbOPO₄ and NbOPO₄ oxides are discussed.

Crystal structure of alluaudite-type Na₄Co(MoO₄)₃

The title compound, tetra­sodium cobalt(II) tris­[molyb­date(IV)], was prepared by solid-state reactions. The structure is isotypic with Na₃In₂(AsO₄)₃ and Na₃In₂(PO₄)₃. The main structural feature is the presence of infinite chains of edge-sharing X₂O₁₀ (X = Co/Na) dimers, which are linked by MoO₄ tetra­hedra, forming a three-dimensional framework enclosing two types of hexa­gonal tunnels in which Na⁺ cations reside. In this alluaudite structure, Co and Na atoms are located at the same general site with occupancies of 0.503 (5) and 0.497 (6), respectively. The other three Na and one of the two Mo atoms lie on special positions (site symmetries 2, -1, 2 and 2, respectively). The structure is compared with similar structures and other members of alluaudite family.