Electrochromic mechanism in a-WO3-y thin films

The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm(-1) due to vibrations of the W6+-O and W6+=O bonds, respectively, and a weaker peak at 220 cm(-1) that we attribute to the W4+-O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to W5+-O and W5+=O bonds appear at 330 and 450 cm(-1), respectively. Comparison of the Raman spectra of sputtered isotopic a-(WO3-y)-O-16 and a-(WO3-y)-O-18 films confirms these assignments. We conclude that the as-deposited films contain mainly the W4+ and W6+ states, and the W5+ states are generated as a result of reduction of the W6+ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states. (C) 1999 American Institute of Physics. [S0003-6951(99)02802-8].