Size-dependent pressure stability of TiO₂ nanoparticles. The average transition pressures of the three phase transition regimes are shown. Nanocrystals of size < 10 nm undergo PIA and remain amorphous (a-TiO₂) upon further compression and decompression. Approximately 12–50 nm crystallites transform to m-TiO₂ upon compression, which then transform to o-TiO₂ on decompression. Coarser crystallites transform directly to o-TiO₂. Reprinted with permission from Ref. [15], Copyright (2006) by the American Physical Society.