Effect of crystallite size on the low-temperature solid-solid phase transformations in the WO₃ system

Ferroelectric nanostructured ε phase tungsten trioxide (ε-WO₃) is emerging as an important functional material with unique gas sensing and optoelectronic properties. However, ε-WO₃ phase is typically unstable in the bulk at room temperature (RT) and it has only been produced at ambient conditions by rapid solidification processes, such as Flame Spray Pyrolysis (FSP). This work aims to assess the overall stability of the ε phase at the nanoscale by studying its transformation from the stable γ phase to upon cooling. In-situ Raman spectroscopy has explored the effect of crystallite size on the solid-solid phase transformation from the symmetric γ (monoclinic) phase into the acentric ε (monoclinic) phase. The phase transformation onset temperature (T_onset) ranges from −43 °C for bulk state to −150 °C for crystallites of 68 nm in size and varies linearly with the inverse of the crystallite size radius (1/r).