X-ray diffraction, extended x-ray absorption fine structure and Raman spectroscopy studies of WO₃ powders and (1-x)WO_3-y·xReO₂ mixtures

Pure ground tungsten trioxide WO₃ and (1-x)WO_3-y·xReO₂ mixtures were studied by x-ray absorption spectroscopy, x-ray powder diffraction and Raman spectroscopy in comparison with hydrogen bronzes H_xWO₃ 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 W⁵⁺ ions, located at the surface of freshly ground powder. The (1-x)WO_3-y·xReO₂ mixtures were found to be composed of monoclinic/orthorhombic WO₃ and orthorhombic ReO₂ phases with a grain boundary containing reduced W^5.7+ ions which are mainly responsible for the compound color at low rhenium ion concentrations. In both cases, the W^(6-z)+ (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 (1-x)WO_3-y·xReO₂ mixtures are proposed and discussed.