Unveiling the crystallographic origin of mechanochemically induced monoclinic to triclinic phase transformation in WO3

This study reports the anomalously stabilized low-temperature triclinic phase (delta-WO3) during high energy ball milling (HEBM) of the monoclinic gamma-WO3 phase. The transformation evidenced unit cell contraction with distorted structural parameters resulted in a lower symmetry phase. The phase transformation was understood to be HEBM-induced pressure/compression involved atomic displacement and octahedral tilting-driven polymorphic transition. The major distortion/cooperative octahedral tilting occurred on the (020) plane of gamma-WO3 parallel to the b axis. The crystallographic origin of such distortions and unit cell compression mechanisms are explained further. The obtained delta-WO3 phase was stable at room temperature owing to its nanocrystalline structure (11 +/- 6 nm) that limits reversible and forward transformations of delta-WO3 -> gamma-WO3 and gamma-WO3 -> epsilon-WO3, respectively.