(NH4)(2)(VO)(VP)O-7, A LAYERED STRUCTURE COMPRISING TETRAHEDRAL VPO7 GROUPS

The new mixed-valence ammonium vanadophosphate (NH4)(2)(VO)(VP)O-7 (di ammonium phosphorus divanadium octaoxide) is isostructural with K2VO(X (2)O(7)) (X = P, V). Its structure consists of [V2PO8](infinity) layer s of corner-sharing VO5 pyramids and VPO7 groups, interleaved with NH4 + ions. The unusual features of this structure are the existence of th e VPO7 groups involving VO4 and PO4 tetrahedra, and the simultaneous p resence of (VO5)-O-IV and (VO4)-O-V polyhedra, observed here for the f irst time in this class of compounds. The VPO7 groups are statisticall y distributed in the framework. Magnetic measurements and bond-valence calculations confirm the mixed-valence character of this phase and sh ow that the VO5 pyramids and VO4 tetrahedra are occupied by V-IV and V -V, respectively.