Symmetry axes can only have the multiplicities 1,2,3,4 or 6 when translational symmetry is present in three dimensions. If, for example, fivefold axes were present in one direction, the unit cell would have to be a pentagonal prism space cannot be filled, free of voids, with prisms of this kind. Due to the restriction to certain multiplicities, symmetry operations can only be combined in a finite number of ways in the presence of three-dimensional translational symmetry. The 230 possibilities are called space-group types (often, not quite correctly, called the 230 space groups). [Pg.20]

Fig. 7.33 (el Unit cell of the 1-2-3 superconductor, orthorhombic, space group Pmtnm. Qne-dirnensional CuOi chains run along the b axis, and two-dimensional CuO, layers lie in the ab plane, (b) The cubic stricture of perovskite. SrTiO,. Three unit cells are shown sleeked vertically, (c) The unit cell of the 1-2-3 superconductor in the context of the surrounding crystal. Copper atoms are surrounded either by five oxygen atoms in a square pyramid or four oxygen atoms in a square plane. (From Holland. C. F. Stacy, A. M. Ace. Chem. Res. 1988,21, 8-15. Reproduced with permission.]... [Pg.155]

Phenazine (39) is found in two crystalline modifications, a- and jS-(Herbstein and Schmidt, 1955a). The a-form (space group P2ja, with two molecules in the unit cell) has been studied at room temperature (Herbstein and Schmidt, 1955a, b) and at about 90°K (Hirshfeld and Schmidt, 1957). The more accurate low-temperature analysis using partial three-dimensional data indicates that the deviations from the expected mmm molecular symmetry only exceed the estimated experimental accuracy ( + 0-003 A) in a direction normal to the mean molecular plane, the maximum deviation being 0-010 A. This slight non-planarity of the molecule is ascribed to the action of intermolecular forces. [Pg.232]

© 2019 chempedia.info