On finite-size effects in the simulation of high pressure, quartz-like structures

Abstract

The stability of two recently suggested crystalline structures arising from compressing quartz is investigated. One phase, called quartz II, is obtained by compressing a model of quartz to pressure P slightly above 22 GPa. The second phase is obtained by shock compressing the quartz II sample to P = 50 GPa. The new structure is a modulated structure of quartz II and the corresponding phase is called quartz IIm. We observe strong size effects or compatibility effects above 26 GPa. If the initial system consists of 360 or 720 SiO2 units, an ‘ideal’ quartz II structure remains stable up to 30 GPa, whereas a larger system of 864 SiO2 units shows modulations of the quartz II structure when compressed above 28 GPa. On decompression from the 50 GPa sample, the larger system appears to be more stable than the smaller system, for which the incompatibility of the intrinsic modulation with the box geometry apparently leads to large fluctuations of the cell geometry.