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Structure of a new high pressure phase in alpha -quartz determined by molecular dynamics studies.

Kingma et al. have recently reported the diffraction evidence of a new phase transition in α-quartz at 21 GPa. We use molecular dynamical calculations to show this phase to be the precursor on the way to amorphization. The powder X-ray patterns generated from the simulated structure show the emergence of superlattice peaks at ≅20 GPa. These predicted diffraction patterns are in excellent agreement with the experimental results and are indicative of a tripling of the unit cell in the a-b (basal) plane of α-quartz. In the pressure amorphized phase the calculated structure factor, S(Q), is also in excellent agreement with the experimental results of Meade et al.