Synchrotron XRD data obtained during compression and decompression of TiO₂ nanoparticles. (a) PIA takes place in 8 nm particles at ∼ 20 GPa (left column). The amorphous phase is recovered under ambient conditions, but subtle changes in the XRD pattern, including increased ordering at low pressures, are observed upon decompression (right column). (b) Integrated XRD data of 8 nm particles during compression or decompression runs. Anatase (t-TiO₂) reflections are indicated by up arrows; arrows labeled “o” and “g” represent o-TiO₂ and Au diffraction positions at 0 GPa. (c) Integrated XRD data of 4 nm particles obtained during decompression. Calculated XRD spectra (for wavelength λ = 0.3344 Å) of o-TiO₂ and m-TiO₂ are given for comparison. The 0 GPa (outside the DAC) spectrum of pressure-amorphized 8 nm particles exhibits diffuse scattering background characteristic of a highly disordered material, with perhaps structural similarity to o-TiO₂. (d) Electron diffraction, however, confirms the amorphous nature of the material at a length scale of that of a crystalline unit cell (∼ 1 nm). Reprinted with permission from Ref. [5], Copyright (2006) by the American Physical Society.