Crystalline post-quartz phase in silica at high pressure - art. no. 155503

alpha -quartz. which has been reported to undergo pressure-induced amorphiz ation, was found to transform to a monoclinic, crystalline phase when compr essed to 45 GPa at room temperature in a close to hydrostatic, helium press ure medium. The x-ray powder diffraction data obtained could be indexed bas ed on a monoclinic cell, and the intensities are in agreement with a P2(1)/ c model structure built up of 3 x 2 zigzag chains Of SiO6 octahedra. This n ew polymorph of silica. which is metastable under ambient conditions., has been isolated for the first time and is one of several possible competing d ense forms containing octahedrally coordinated silicon.