Raman spectroscopic studies of electrochromic a-WO3

We report on the effect of electrochromic coloration on the Raman spectra of evaporated and sputtered a-WO3-y films. 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 vibrations of the W4+-O bonds. When lithium or hydrogen ions 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 a-(WO3-y)-O-16 and a-(WO3-y)-O-18 isotopic 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 ions are inserted. We propose that the electrochromism in a-WO3-y films originates from reduction of the W6+ states to W5+ due to the double insertion/extraction of ions and electrons and the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states. (C) 1999 Elsevier Science Ltd. All rights reserved.