A niobium phosphate bronze closely related to the ITB tungsten bronzes: K7Nb14+ xP9− xO60 (x= 0.13)

Abstract

A new niobium phosphate bronze K7Nbi4+*P 9 -x0 6. has been isolated and its structure has been determined by X-ray diffraction from a single crystal. It crystallizes in the orthorhombic system, space group Pmma, with a = 36.883(3) A, b = 10.603(l) A and c = 6.4526(5) A. The mixed framework of this bronze is closely related to the intergrowth tungsten bronzes (ITB): it consists of [Nb3P201,], layers deduced from the ITB [Mo,O,& layers observed in Sb2Mok,0jl by replacing two octahedra out of five by PO4 tetrahedra. Four such layers are stacked along a, sharing the comers of their polyhedra, forming [Nb,,PsO& slabs connected through [NbP,Os], chains related to the perovskite and to the hexagonal tunnels. This host lattice determinates five types of tunnels of the brownmillerite and of the hexagonal tungsten bronzes, where the K+ ions are located.