Pressure-induced amorphization and a possible polyamorphism transition in nanosized TiO2: An x-ray absorption spectroscopy study

The phenomenon of grain-size dependent pressure-induced amorphization (PIA) in TiO2 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- TiO2 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 TiK -edge. The local environment of the cation, to within a few nearest-neighbor shells, has been monitored up to ∼30 GPa where the HDA phase is stabilized. In this phase the titanium atom is surrounded by 3±0.5 oxygens at 1.89 Å and 3±0.5 oxygens at 2.07 Å. 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 (α -PbO2 -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 α -PbO2 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±0.5 oxygens at 1.84 Å and 2.5±0.5 oxygens at 2.06 Å 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.