Pressure-induced polyamorphism in TiO2 nanoparticles

Two different nanometric (6 nm) ${\text{TiO}}_{2}$ compounds, anatase polycrystals and amorphous particles, were investigated under high pressure using Raman spectroscopy. Nanoanatase undergoes a pressure-induced amorphization. The pressure-induced transformations of this mechanically prepared amorphous state are compared with those of a chemically prepared amorphous particles. In the mechanically prepared amorphous state, a reversible transformation from a low-density amorphous state to high-density amorphous state (HDA1) is observed in the range 13--16 GPa. In the chemically prepared sample, a transformation to a new high-density amorphous state (HDA2) is observed at around 21 GPa. Further compression leads to the transformation $\text{HDA}2\ensuremath{\rightarrow}\text{HDA}1$ at $\ensuremath{\sim}30\text{ }\text{GPa}$. We demonstrate that depending on the starting amorphous material, the high-pressure polyamorphic transformations may differ. This observation indicates that pressure is a suited tool to discriminate between nanomaterials apparently similar at ambient conditions.