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 P3(1)21) of SiO2 and GeO2 were refined from single-crystal X-ray diffraction data under hydrostatic pressures up to 10.2 GPa for SiO2 and 5.57 GPa for GeO2. R(w)(F) values range from 2 to 5%. Hexagonal unit-cell parameters for SiO2: a = 4.921(1), c = 5.4163(8) angstrom at ambient conditions; a = 4.604(1), c = 5.207(1) angstrom at 10.2(1) GPa. For GeO2: a = 4.9844(2), c = 5.6477(2) angstrom at normal pressure; a = 4.750(1), c = 5.548(5) angstrom at 5.57 GPa. Volume decrease, 16% for SiO2 and 11% for GeO2 is accomplished mainly by tetrahedral tilting, the rest arising from tetrahedral angle distortion. GeO2-quartz is an almost perfect high-pressure model for SiO2-quartz: At 10 GPa the geometry of the SiO2-quartz structure approaches that of GeO2 at ambient pressure (e.g. similar values for c/a, atomic parameters, tetrahedral tilt angle, tetrahedral distortion). These values then further change for GeO2 with increasing pressure reflecting increasing structural distortion. The Si-O-Si angle decreases with pressure from 144.2(2) to 130.3(1)-degrees, the Ge-O-Ge angle from 130.0(1) to 123.4(3)-degrees. The Si...Si distance between vertex-connected tetrahedra shrinks from 3.0627(4) to 2.9152(8) angstrom, the respective Ge...Ge distance from 3.1515(3) to 3.054(1) angstrom. Both distances, at maximum pressures, fall slightly below smallest reported values for silicates and germanates at ambient conditions. The variations in the angle T-O-T and the nonbonded distance T...T are nearly independent of changes in the T-O bond lengths. The shortest O...O distance between unconnected tetrahedra decreases from 3.345(2) to 2.793(2) angstrom in SiO2, and from 3.023(3) to 2.809(9) angstrom in GeO2. For quartz this distance remains longer than the longest tetrahedral edge, which increases from 2.640(3) to 2.690(2) angstrom. For GeO2 however, it shrinks to the second-shortest oxygen-oxygen distance in the structure; only two symmetry equivalent tetrahedral edges are shorter, 2.725(4) angstrom, at the maximum pressure. The two symmetry equivalent second-shortest intertetrahedral O...O distances in GeO2 decrease from 3.192(2) to 2.926(6) angstrom, becoming shorter than the largest tetrahedral edge, which increases from 2.903(2) to 2.943(5) angstrom. Extrapolating these developments, the oxygen atoms would reach positions at the lattice points of a cubic body-centred lattice (Sowa, 1988). Increasing tetrahedral distortion with pressure is displayed mainly by angle distortion. The quantity, DI(OTO) (Baur, 1974), increases from 0.6 to 2.6% for SiO2 and from 2.5 to 4.2% for GeO2. The longer Si-O bond remains constant at 1.614(2) angstrom, whereas the shorter one decreases from 1.605(2) to 1.599(2) angstrom. These values reflect the stiffness of the respective bonds. The changes in the Ge-O bond lengths are correspondingly small but are less regular with increasing pressure. The variations of bond angles O-T-O and respective tetrahedral edges are nearly independent of changes in the T-O bond lengths. Tetrahedral distortion mechanisms are seen to differ in comparing changing pressure or temperature.