STRUCTURAL INVESTIGATION OF P4W24O80 - A NEW MONOPHOSPHATE TUNGSTEN BRONZE

The crystallographic study of the tungsten phosphate bronze P4W24O80 w as performed from three-dimensional single-crystal X-ray diffraction d ata and electron microscopy. This compound crystallizes in the orthorh ombic system with unit-cell dimensions a = 5.312 (1), b = 6.5557 (8), c = 42.196 (8) Angstrom and space group P2(1)2(1)2(1). The crystal str ucture was solved by direct methods and Fourier techniques, and refine d to the reliability factor R = 0.0284 (wR = 0.0272). Its determinatio n confirms that it belongs to the monophosphate tungsten bronze family of the general formula (PO2)(4)(WO3)(2m) with the value m = 12. Large empty cages surrounded by 18 O atoms are built up of eight WO6 octahe dra and four PO4 tetrahedra sharing corners. They are located between two WO3-type slabs forming pentagonal-shaped tunnels running in the a direction. Electron microscopy investigations confirm that the studied crystal does not imply modulation phenomena when other crystals of th e same composition exhibit satellite reflections with a modulation vec tor q involving a doubling of a. The observations also reveal the exi stence of a monoclinic form of the m = 12 compound, which is a regular intergrowth of m = 11 and m = 13 members in a similar way to the m = 5 member of the series where the same feature has already been observe d. On each edge of a WO3-type slab, a large variation (from 1.73 to 2. 09 Angstrom) of the six W-O distances within the WO6 octahedron is not ed, which yields an oxidation state of W near 6, whereas for the WO6 o ctahedra located in the middle part of the slab the six W-O distances are gathered about their mean value (1.92 Angstrom), which involves a more important electronic delocalization. The thermal motion of the W atoms is described. The absolute structural configuration is tested on the basis of some calculated structure factors, which are more sensit ive to the x, y, z --> -x, -y, -z change of atomic positions.