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A study of the phase transitions in WO3 crystals by Raman and absorption spectroscopy reveals that WO3 possesses two stable (or metastable) modifications at room temperature, differing slightly but definitely in their physical properties. Measurements of electrical conductivity show that both are insulators at low temperatures and semiconductors at higher temperatures. One is monoclinic and the other triclinic. Both exhibit at lower temperatures discontinuities in their physical properties without accompanying changes in cell parameters.
Salje et al. (Thu,) studied this question.