Phase Transitions in Gas-Evaporated WO3Microcrystals: A Raman Study

Temperature dependence of Raman scattering from gas-evaporated WO 3 has been investigated. From a comparison between the present Raman data and that of the bulk crystals, it is shown that the phase transition behaviors of the microcrystals are completely different from those of the bulk crystals. At room temperature, the possible phase of the microcrystals is the mixture of phases V and VII of the bulk crystals, which is supported by the X-ray diffraction measurements. At around 230 K, the microcrystals gradually undergo a phase transition, transforming into a new phase not reported so far or a mixture of phase VII and the new phase. Possible explanations for the phase mixture based on the size dependence of the Raman spectra are given.