STRUCTURAL-PROPERTIES OF ALPHA-QUARTZ UNDER HIGH-PRESSURE AND AMORPHIZATION EFFECTS

The behavior of crystalline alpha-quartz under hydrostatic pressure is studied via total-energy calculations based on the full-potential lin earized augmented-plane-wave method, within the local-density approxim ation. By examining recent experimental data and our theoretical resul ts, the structural properties of alpha-quartz near and above the trans ition to the amorphous state are determined. Our results indicate that the SiO4 tetrahedra distort, with a regular and slight decrease of th e mean distance Si-O; the interpolyhedral Si-O-Si angles and the O-O d istances decrease below the smallest value experimentally observed on ordered silicates, with a dramatic reduction of the structural voids. The oxygen lattice tends towards an ideal close-packed configuration, perhaps highly instable.