In search of the crystal structure of low quartz

Abstract

AbstractA comparison of numerous determinations of unit cell dimensions and of crystal structure refinements of low quartz, SiO2, shows that there exists no study at a defined temperature and at ambient pressure of a chemically well characterized sample, for which we have precise unit cell lengths and at the same time positional coordinates of high precision for the silicon and oxygen atoms. An analysis of the available data from 18 carefully selected measurements of cell lengths and 25 determinations of positional coordinates results in an averaged structure at 291 K in space groupP3121 with unit cell dimensions:a= 4.9130(1) Å,c= 5.4047(1) Å; and standardized coordinates:x(Si) = 0.5301(2),x(O) = 0.4139(5),y(O) = 0.1466(4) andz(O) = 0.1188(3); the silicon atom is in Wyckoff position 3a (x, 0, 1/3) and the oxygen atoms are in Wyckoff position 6c (x, y, z). Error estimates for cell dimensions and coordinates are taken as the mean of the observed e.s.d.‘s in the various original structure refinements. The bond lengths within one coordination tetrahedron are, twice each, Si–O,a 1.604(2) Å and Si–O,b 1.613(2) Å; the angles around the silicon atom are O,a–Si–O,a 109.0(1)°, O,b–Si–O,b 109.5(1)°, and twice each O,a–Si–O,b 110.4(1)° and O,b–Si–O,a 108.8(1)°; the Si–O–Si angle between two tetrahedra is 143.7(1)°. For the time being this is the best currently available estimate of the geometry of the crystal structure of low quartz.