Pressure-induced amorphization and a possible polyamorphism transition in nanosizedTiO2: An x-ray absorption spectroscopy study

Abstract

The phenomenon of grain-size dependent pressure-induced amorphization (PIA) in ${\text{TiO}}_{2}$ nanomaterials has been evidenced by several experiments in recent years. This has stemmed mainly from x-ray diffraction (XRD) and Raman studies of ultrafine grained anatase. Until now there is no experimental evidence of the length scale of disorder nor is there a clear picture of the amorphous structures, specifically in the case of pressure amorphised anatase-${\text{TiO}}_{2}$ starting material. The unresolved issues of the structural details and atomic-scale picture of the high-density amorphous (HDA) phase have now been addressed in an x-ray absorption spectroscopy (XAS) pressure study at the $\text{Ti}\text{ }K$-edge. The local environment of the cation, to within a few nearest-neighbor shells, has been monitored up to $\ensuremath{\sim}30\text{ }\text{GPa}$ where the HDA phase is stabilized. In this phase the titanium atom is surrounded by $3\ifmmode\pm\else\textpm\fi{}0.5$ oxygens at $1.89\text{ }\text{\AA{}}$ and $3\ifmmode\pm\else\textpm\fi{}0.5$ oxygens at $2.07\text{ }\text{\AA{}}$. The XAS results of this study suggest that a precursor ordered structural phase, different to that of anatase, is prevalent before amorphization occurs. The nature of this high-pressure stabilized precursor to amorphization likely depends on the starting experimental conditions at ambient pressure. In some cases this precursor has been identified as the columbite ($\ensuremath{\alpha}{\text{-PbO}}_{2}$-type) crystalline structure perhaps with only limited range order. Samples of this type appear to evidence a ``memory effect'' in that after cycling into the HDA phase (up to 30 GPa) where complete structural disorder prevails, this $\ensuremath{\alpha}{\text{-PbO}}_{2}$ structural intermediate is reestablished in a limited pressure range of the decompression cycle. It is also found that a new structure is stabilized in all cases of samples decompressed from the HDA phase to ambient conditions, characterized by fivefold coordinated Ti ($2\ifmmode\pm\else\textpm\fi{}0.5$ oxygens at $1.84\text{ }\text{\AA{}}$ and $2.5\ifmmode\pm\else\textpm\fi{}0.5$ oxygens at $2.06\text{ }\text{\AA{}}$) and is therefore structurally distinguishable from the HDA phase. These conceptual pictures are derived from the pressure dependence of both the extended x-ray-absorption fine structure (EXAFS) and the preedge parts of the absorption spectra.