RIETVELD REFINEMENT OF POWDER X-RAY DIFFRACTION OF NANOCRYSTALLINE NOBLE METALS - TUNGSTEN TRIOXIDE

摘要

Tungsten trioxide powders were prepared from peroxo-tungstic acid (PTA) precursor obtained by sol-gel route, in aqueous medium. Two WO_3 samples were doped with noble metals, by "loading" them with silver and metallic palladium. The unit cell parameters of the WO_3 monoclinic phase, the effective crystallite mean size, D_(eff) (nm), and the mean root mean square (rms) of the microstrains , <ε~2>1/2hk1 ,were calculated by Rietveld refinement analysis. The distorsion of the WO_3 crystal structure has been used by the ISODISPLACE software tool to transform the P2_1 structure of WO_3 on to the distortion-mode basis to visualize and quantify the displace modes. The atomic coordinate type has been used to compute three-dimensional structure model as the Radial Distribution Functions g(r) for a 3000-atom model of WO_3-Ag and WO_3-Pd systems, which reflects the space correlations between W-O and W-W atoms.