Crystal structures of the low-temperature quartz-type phases of SiO₂ and GeO₂ at elevated pressure

Lattice parameters and crystal structures of the low-temperature quartz-type ('low-quartz’) forms (space group P3121) of Si02 and Ge02 were refined from single-crystal Xray diffraction data under hydrostatic pressures up to 10.2 GPa for Si02 and 5.57 GPa for Ge02. RW(F) values range from 2 to 5%. Hexagonal unit-cell parameters for Si02: a = 4.921(1), c—5.4163(8) A at ambient conditions; a = 4.604(1), c = 5.207(1) A at 10.2(1) GPa. For Ge02: a = 4.9844(2), c = 5.6477(2) A at normal pressure; a = 4.750(1), c = 5.548(5) A at 5.57 GPa. Volume decrease, 16% for Si02 and 11% for Ge02 is accomplished mainly by tetrahedral tilting, the rest arising from tetrahedral angle distortion. Ge02-quartz is an almost perfect high-pressure model for Si02-quartz: At 10 GPa the geometry of the Si02-quartz structure approaches that of Ge02 at ambient pressure (e.g. similar values for c/a, atomic parameters, tetrahedral tilt angle, tetrahedral distortion). These values then further change for Ge02 with increasing pressure reflecting increasing structural distortion. The Si—O—Si angle decreases with pressure from 144.2(2) to 130.3(1)°, the Ge —O —Ge angle from 130.0(1) to 123.4(3)°. The Si.Si distance between vertex-connected tetrahedra shrinks from 3.0627(4) to 2.9152(8) A. © 1992, Walter de Gruyter. All rights reserved.