High Pressure Polymorphism in Silica

Fundamental crystal chemistry and first-principles total-energy calculations are used to examine stable and metastable high-pressure silica structures. We find that a large class of energetically competitive phases can be generated from hcp arrays of oxygen with silicon occupying one-half of the octahedral sites. Calculations for specific structures provide an explanation for a number of recent high-pressure results for crystalline silica and allow us to understand the nature of the short- and intermediate-range order in the high-pressure amorphous state.