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. T hese simulations determine the changes in the potential energy minima upon decreasing volume. The transformations are found to arise from discrete mec hanical instabilities associated with disappearances of local minima on the potential energy surface. The ionic displacements arising from the mechani cal instabilities are localized to a small number of ions, and involve prim arily oxygen motion. The mechanical instabilities appear to be triggered in part by small Si-O-Si angles, which are highly strained. Compression of th e glass also leads to a slight ordering of the oxygen ion packing. (C) 2001 Published by Elsevier Science B.V.