HIGH-PRESSURE 4-COORDINATED STRUCTURE OF SIO2

A triclinic crystalline structure that exists between alpha-quartz and the high-pressure disordered phase at 22 GPa has been revealed from l arge-scale constant-pressure classical molecular-dynamics calculations . This phase, which is stable within a narrow pressure range and trans forms to a disordered structure upon further compression, results from a shear distortion of alpha-quartz but maintains the same four-coordi nated Si-O environment. The theoretical diffraction pattern computed f rom atom coordinates optimized using first-principles pseudopotential plane-wave calculations is in excellent agreement with the published e xperimental data. [S0163-1829(97)07941-1].