A niobium phosphate bronze closely related to the ITB tungsten bronzes: K 7Nb 14+ xP 9- xO 60 ( x = 0.13)
A new niobium phosphate bronze K 7Nb 14+ xP 9- xO 60 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) Å, b = 10.603(1) Å and c = 6.4526(5) Å. The mixed framework of this bronze is closely related to the intergrowth tungsten bronzes (ITB): it consists of [Nb 3P 2O 13] ∞ layers deduced from the ITB [Mo 5O 15] ∞ layers observed in Sb 2Mo 10O 31 by replacing two octahedra out of five by PO 4 tetrahedra. Four such layers are stacked along a, sharing the corners of their polyhedra, forming [Nb 12P 8O 52] ∞ slabs connected through [NbP 2O 8] ∞ 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.