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 alpha-quartz at 21 GPa. We use molecular dynamica l calculations to show this phase to be the precursor on the way to am orphization. The powder x-ray patterns generated from the simulated st ructure show the emergence of superlattice peaks at almost-equal-to 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 alpha-quartz. In the pressure am orphized phase the calculated structure factor, S(Q), is also in excel lent agreement with the experimental results of Meade et al.