Refinement of the atomic parameters of α-quartz

Abstract

The positional and thermal parameters of a-quartz at room temperature have been redetermined accurately using Cu Kc~ diffractometric intensities from a natural crystal showing unusually little primary extinction. Anisotropic refinement on the basis of the l l2 observable independent T_Fol' S was performed by least squares and differential synthesis to final R values of 3-6 and 3.3 ~ respectively. In good agreement with the results of YoLmg & Post, the positional parameters are found to be x=0. 4145, y=0-2662, z=0.1189, u=0.4698. The Si-O bond lengths are 1.597 and 1.617 ( + 0"003) A, the O-O bond lengths range from 2.604 to 2.640 ( + 0.002 to 0-005) A, and the Si-O-Si angle is 144.0 ( + 0.2)% The dimensions and orientation of the thermal vibration ellipsoid of silicon agree well with the findings of Young & Post, the maximum r.m.s, vibrational amplitude, 0.080 .~, being parallel to c. For oxygen the agreement is not definite. The present study indicates that the maximum r.m.s, displacement, 0.132 A, is perpendicular to the plane defined by a given oxygen atom and the two silicon atoms which it links.