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

Ferroelectric nanostructured epsilon phase tungsten trioxide (epsilon-WO3) is emerging as an important functional material with unique gas sensing and optoelectronic properties. However, epsilon-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 epsilon phase at the nanoscale by studying its transformation from the stable gamma phase to upon cooling. In-situ Raman spectroscopy has explored the effect of crystallite size on the solid-solid phase transformation from the symmetric gamma (monoclinic) phase into the acentric epsilon (monoclinic) phase. The phase transformation onset temperature (Tonset) ranges from -43 degrees C for bulk state to -150 degrees C for crystallites of 68 nm in size and varies linearly with the inverse of the crystallite size radius (1/r).