It was confirmed that the divalent cations, Zn 2+ and Cu 2+ were electrochemically inserted into amorphous WO 3 films prepared by r.f.-magnetron sputtering method. Coloration of the films was also observed, and the films were, however, bleached gradually. The bleaching mechanism was discussed based on the ab-initio molecular orbital calculations, in which Li + or Zn 2+ ions were introduced into the WO 3 frameworks. In the case of Zn insertion, electrons were trapped in a non-bonding Zn 4s

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... l just below the conduction band. It was finally concluded that in the films with distorted WO 3 frameworks, Zn 4s orbital was mingled in the conduction band and electrons were free to be transferred between W 6+ and W 5+ . As the structural relaxation progressed, electrons were gradually trapped in Zn 4s orbital, and the films were finally bleached.