X-ray diffraction, extended x-ray absorption fine structure and Raman spectroscopy studies of WO3 powders and, (1-x)WO3-y center dot xReO(2) mixtures

Pure ground tungsten trioxide WO3 and (12x)WO32y xReO2 mixtures were studied by x-ray absorption spectroscopy, x-ray powder diffraction and Raman spectroscopy in comparison with hydrogen bronzes HxWO3 and hydrogenated calcium tungstate CaWO4:H. It was found that a grinding of pure WO3 leads to a decrease of the crystallites size and a development of the bluish coloration. The color change was found to be reversible under moderate heat treatment or after storage in oxidizing atmosphere and is attributed to the reduced W51 ions, located at the surface of freshly ground powder. The (12x)WO32y xReO2 mixtures were found to be composed of monoclinic/orthorhombic WO3 and orthorhombic ReO2 phases with a grain boundary containing reduced W5.71 ions which are mainly responsible for the compound color at low rhenium ion concentrations. In both cases, the W(62z)1 (0,z<1) color centers are responsible for strong optical absorption resulting in the dramatic decrease of the total Raman intensity. The structural models of free surface in pure ground WO3 and bulk WO3 /ReO2 intragrain boundary in (12x)WO32y xReO2 mixtures are proposed and discussed.