A NEW PHASE AND PRESSURE-INDUCED AMORPHIZATION IN SILICA

Using first principles variable cell shape molecular dynamics, we pred ict a new structure for silica. This structure results from annealing alpha-quartz at pressures near a major phonon instability. The new pha se is obtained by rotations of SiO4 tetrahedra producing edge-sharing SiO6 octahedra and SiO5 cuboids. This type of mechanism was proposed t o account for structural transitions in silicate melts and amorphous s olids under pressure. Here this mechanism is observed in a crystalline to crystalline phase transformation for the first time. The diffracti on pattern of the new phase compares favorably with that of the uniden tified intermediate crystalline phase found in silica during the amorp hization.