Distribution over Space Groups

In the analysis of space group frequencies, one should be aware of the biases in the CSD. For example, in crystalline oxohydrocarbons a predominance of P2] and P2]2i2] was detected [4], but this is because many of the compounds are derivatives of sugars or other natural products, studied in their naturally occurring, enan-tiomerically pure form, so that the choice of a chiral space group is forced. 

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Se Sj2 CS2 and CS crystallize in the trigonal space group R3m. The molecular geometry ot he twelve-membered selenium sulfide ring molecule resembles that of Si2. The structure, however, is also disordered. The distribution of sulfur and selenium over the atomic sites is shown in Fig. 2. 

A crystal is not, in actual fact, a simple array of points, each of which is a pattern-unit. In the first place, an atom is not a point its electrons, which scatter the X-rays, are distributed over distances commensurable with the interatomic distances. Secondly, each pattern-unit in a crystal often consists, not of one atom, but a group of atoms. The pattern-unit is thus not a point, but has a diffuse and often irregular form. However, for the purpose of diffraction theory, as far as it is carried in this chapter, the diffuse pattern-unit may be mentally replaced by a point. It will be shown in Chapter VII that the form of the pattern-unit affects the intensities of the diffracted beams but it does not affect their positions, which depend only on the space-lattice, the fundamental arrangement of identical pattern-units. 

The Cu occupy one set of tet interstices in y-CuI and, on heating, Cul undergoes a phase transition to the P phase at 642 K. The structure of P-Cul can be described as an hexagonal dose-packed (hep) sublattice of anions, with the Cu distributed in the approximate ratio 5 1 over two sets of tet interstices within space group Pi ml [20]. 

Li2MCl4 compounds undergo a phase transition to a disordered cation-deficient rocksalt-structured form (space group Fm3m), in which the two cation species are randomly distributed over all the oct sites, which then have an average occupancy of 4 [132, 133]. 

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