On the Phase Transition of Tungsten Trioxide below Room Temperature

Abstract

Microscopic observation, thermal analysis, measurement of electrical resistivity and X-ray study of WO 3 were carried out in relation to the phase transition from room temperature down to -70°C. The crystal of WO 3 is monoclinic ( a =7.30Å, b =7.53Å, c =7.68Å, β=90°54') at room temperature, and it transforms to triclinic ( a =7.30Å, b =7.52Å, c =7.69Å, α=88°50', β=90°55', γ=90°56') near 17°C, and transforms to monoclinic ( a =5.27Å, b =5.16Å, c =7.67Å, β=91°43') near -40°C, on cooling. These two transitions could be clearly observed in the single-domain crystal in the room temperature phase, but the phase transitions of complicated appearance were observed in the multidomain crystal owing to a small difference between free energies in the room-temperature phase and the triclinic phase. Both electrical resistivity versus temperature curve and thermal differential curve show a slight anomaly at 17°C transition and a remarkable anomaly at -40°C transition.