Structural transformations in silica glass under high pressure

The structural transformations in silica glass that lead to changes in ion coordination under high pressure are examined with molecular simulations. These simulations determine the changes in the potential energy minima upon decreasing volume. The transformations are found to arise from discrete mechanical instabilities associated with disappearances of local minima on the potential energy surface. The ionic displacements arising from the mechanical instabilities are localized to a small number of ions, and involve primarily oxygen motion. The mechanical instabilities appear to be triggered in part by small Si–O–Si angles, which are highly strained. Compression of the glass also leads to a slight ordering of the oxygen ion packing.