Structural Investigation of P₄W₂₄O₈₀: A New Monophosphate Tungsten Bronze

The crystallographic study of the tungsten phosphate bronze P₄W₂₄O₈₀ was performed from three-dimensional single-crystal X-ray diffraction data and electron microscopy. This compound crystallizes in the orthorhombic system with unit-cell dimensions a = 5.312 (1), b = 6.5557 (8), c = 42.196 (8) Å and space group P2₁2₁2₁. The crystal structure was solved by direct methods and Fourier techniques, and refined to the reliability factor R = 0.0284 (wR = 0.0272). Its determination 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 octahedra 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 the same composition exhibit satellite reflections with a modulation vector q* involving a doubling of a. The observations also reveal the existence 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 observed. On each edge of a WO₃-type slab, a large variation (from 1.73 to 2.09 Å) of the six W—O distances within the WO₆ octahedron is noted, which yields an oxidation state of W near 6, whereas for the WO₆ octahedra located in the middle part of the slab the six W—O distances are gathered about their mean value (1.92 Å), 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 sensitive to the x, y, z → −x, −y, −z change of atomic positions.

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