(NH₄)₂(VO)(VP)O₇, a Layered Structure Comprising Tetrahedral VPO₇ Groups

The new mixed-valent ammonium vanadophosphate (NH₄)₂(VO)(VP)O₇ (diammonium phosphorus divanadium octaoxide) is isostructural with K₂VO(X₂O₇), (X = P, V). Its structure consists of [V₂PO₈]_∞ layers of corner-sharing VO₅ pyramids and VPO₇ groups, interleaved with NH₄⁺ ions. The unusual features of this structure are the existence of the VPO₇ groups involving VO₄ and PO₄ tetrahedra, and the simultaneous presence of V^IVO₅ and V^VO₄ polyhedra, observed here for the first time in this class of compound. The VPO₇ groups are statistically distributed in the framework. Magnetic measurements and bond-valence calculations confirm the mixed-valence character of this phase and show that the VO₅ pyramids and VO₄ tetrahedra are occupied by V^IV and V^V, respectively.

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