HIGH-PRESSURE PHASES IN THE SYSTEM W-O .2. STRUCTURE DETERMINATION OF WO2.625 BY HRTEM AND X-RAY-POWDER DIFFRACTION ANALYSIS

A new type of tungsten oxide has been synthesized from a mixture of W and WO3 by a solid-phase sintering method under high-pressure conditions. The crystal structure of the new oxide was investigated by HRTEM, selected-area electron diffraction and X-ray powder diffraction. The structure belongs to space group Pbam or P2(1)2(1)2 and has the following unit-cell parameters: a = 21.431(9), b = 17.766 (7), c = 3.783 (2) angstrom, v = 1440 angstrom, Z = 32, D(x) = 8.33 g cm-3. The structural model and W-cation positions were determined by HRTEM and image processing. X-ray powder analysis and the SHELX computer program were used to prove the proposed structural model: N = 158, R = 0.075, U(iso)(W) = 0.019 (3), U(iso)(O) = 0.055 (12) angstrom2. The investigated crystal structure is, in fact, similar to WO2.72 and is formed by W-O octahedra and pentagonal bipyramids.