Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 78

A working knowledge of point groups and space groups is invaluable for anyone interested in crystal chemistry and structure‐property relations. Without such knowledge, one is entirely dependent for information about crystal structures on the availability of 3D models, crystal structure libraries, or good quality drawings in the literature. Once the basic principles of space groups are grasped, however, it is possible to make drawings of a structure for oneself, from different orientations if required, or even to construct one's own 3D models. All that is needed is a listing of the atomic coordinates in the structure and details of the relevant space group. There are also various excellent commercial software packages, such as CrystalMaker, available which take away much of the hard work involved in preparing structural drawings.

This section is written for the non‐crystallographer; short‐cuts are made and many of the complications and subtleties of space groups are avoided. The main objective is to show the relation between space groups and sets of atomic coordinates on the one hand and 3D crystal structures on the other. A familiarity with unit cells, crystal systems, Bravais lattices and the elements of point and space symmetry, summarised in Sections 1.1–1.6, is essential background knowledge. We begin with a discussion of point groups. Point groups are much simpler than space groups because the elements of translational symmetry are absent from point groups. They therefore provide a relatively painless introduction to the subject while introducing the necessary concepts at an early stage. We then look at a few selected examples of space groups and finish with two examples that show the derivation of crystal structures from space group and atomic coordinates information.