Monophosphate tungsten bronzes with pentagonal tunnels (PO 2) 4(WO 3) 2 m: Structure of two even- m members P 4W 12O 44 ( m = 6) and P 4W 16O 56 ( m = 8)

Abstract

Crystals of P 4W 12O 44 and P 4W 16O 56 have been synthesized. They are orthorhombic, space group P2 12 12 1 with a = 5.2927(7), b = 6.5604(7), c = 23.549(3)Å for P 4W 12O 44 and a = 5.2943(5), b = 6.5534(4), c = 29.700(4)Å for P 4W 16O 56. Structures were solved by the heavy-atom method and refined to R = 0.033 ( R w = 0.044) with 2031 independent reflections for P 4W 12O 44 and R = 0.043 ( R w = 0.052) with 2272 independent reflections for P 4W 16O 56. The structures are related to that of P 4W 8O 32 and can be described by the association of zig-zag chains of octahedra and tetrahedra. The members of the series (PO 2) 4(WO 3) 2 m differ from one another by the length of the links in the chains of polyhedra. Pentagonal tunnels and O 18 cages are formed at the junction between the ReO 3-type slabs and the slices of PO 4 tetrahedra. The surroundings of P and W have been studied and compared with those of other phosphate tungsten bronzes; the relation with the apparent oxidation state of tungsten is discussed.