X-ray diffraction study on the framework structure of amorphous tungsten trioxide films

Amorphous tungsten trioxide films were prepared from iso-polytungstic acid as a precursor and by RF magnetron sputtering method. Framework structure was investigated based on X-ray diffraction analyses, in which amplitude function as well as radial distribution function was used. And the previously reported films prepared by vacuum evaporation and an ion exchange method using Na₂WO₄·2H₂O as a precursor were also re-examined. It was found that, in the films derived from iso-polytungstic acid, the framework had a similar manner to that in hexagonal-WO₃ crystal, whose framework was constructed by 3-, 4- and 6-membered rings formed by WO₆ octahedra. It was supposed that edge-sharing polyhedra and tungsten and/or WO₆ defects were present in the as-prepared film and changed to corner-sharing connection and continuous framework by post-annealing. It was also suggested that structural water was present in a hexagonal tunnel in the framework. These structural characteristics were also observed in the films prepared by vacuum evaporation and ion exchange methods. As for the sputtered films, formation of two framework-types without structural water was confirmed. The hexagonal-WO₃ like framework was formed in the films prepared under a low O₂ partial pressure, and tetragonal-WO₃ like one with an ordinary ReO₃-type structure made up with 4-membered rings was produced at a high O₂ pressure.