(a) Raman spectra of 6 nm amorphous particles prepared by sol–gel synthesis with increasing pressure. An LDA to a new HDA2 transformation is observed above 16.7 GPa. Above 30.2 GPa, another amorphous state (HDA) appears. (b) Raman spectra of 6 nm amorphous particles with decreasing pressure. The back transformation HDA1–HDA2 is observed in the range of 28.2–25.0 GPa and the transformation HDA2–LDA starts around 11.7 GPa. Reprinted with permission from Ref. [40], Copyright (2010) by the American Physical Society.