Auszug Wegen ihrer hohen Symmetrie weisen die meisten Si02-Modifikationen nul' wenige unabhangige (Si-O)-Abstande auf. Jedoch die Struktur des Coesits ent.halt zwei unabhangige SrOa-Tetraeder, acht (Si-O)-Bindungen, fiinf (Si-O-Si)-und zwolf (O-Si-O)-Winkel und bietet sich somit als ein ausgezeichnetes Objekt zur Priifung del' Theorie del' (Si-O)-Bindungen an. Urn den bestmoglichen Satz von (Si-O)-Abstanden zu erhalten, haben die Verfasser die Struktur eines synthetischen Kristalls bis zu R =

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... 024 verfeinert. Die Gitterkonstanten dieses Kristalls sind: a = 7,135(1), b = 12,372(1), c = 7,173(1), /-] = 120,36 (1) -. Eine Elektronenpopulations-Analyse nach MULLIKENwurde unter Annahme r-iner mit.t.leren Lange von 1,614 A fiir aIle (Si-O)-Bindungen berechnet bei Abstract Because of their high symmetry, most of the silica polymorphs contain only a few nonequivalent Si-O distances. However, the structure of coesite contains t\VO nonequivalent silicate tetrahedra, eight Si-O bonds, five Si-O-Si and t welve O-Si-O angles. thus making it an excellent candidate for testing Si-O bonding theory. In order to obtain the best possible set of Si-O distances, we refined the structure using single-crystal diffraction data from a synthetic crystal (unweighted R = 0.024). Cell parameters for this crystal are a = 7.135(1) A, b = 12.372(1), C = 7.173(1), and f3 = 120.36(1). A Mulliken population analysis was calculated with all Si-0 bond lengths clamped at 1.614 A and using the valence angles determined by ARAKI and ZOLTAI (1969) for which R = 0.076, and those obtained from the present experiment., The Si-0 bond overlap populations calculated using the valence angles ()btained for the new coordinates were found to be more highly correlated (r2 = 0.91 V8. r2 = 0.60 for the older data) with the observed Si-O bond lengths, suggesting that precise structural data are necessary to obtain a nearlinear correlation between bond-overlap population and bond length. The individual Si-O bond lengths to the oxygen in the Si-O-Si linkages in coesite vary (1) non-lineary when plotted against Si-O-Si angle and (2) linearly when plotted against -ljcos 4 (Si-O-Si), shorter bonds being associated w it h wider angles. Shorter bonds also tend to be involved in the wider 0 -Si -C) angles. The results of this experiment show that there is no evidence for departure from stoichiometry, that there is no unusual thermal vibration of tilt> uxygen atom in the linear Si-O-Si linkage, and that variations in individual i-O distances are closely related to calculated bond-overlap populations, to the geminal nonbonded repulsions and to the charge of the oxygen atom. < O-Si-O >3 is a measure of the strength of the bond common to the three angles comprising < O-Si-O >3, larger n(Si-O) values tending to involve bonds with larger < O-Si-O >3 values. Despite the small range of < O-Si-O >3 angles (108.~)-109.9°) exhibited by 1.63 A study of the structural chemistry of coesite 1.63,---------,