Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 29

Crystal structures may be described in various ways. The most common and one which gives all the necessary information, is to refer the structure to the unit cell. The structure is given by the size and shape of the cell and the positions of the atoms, i.e. atomic coordinates, inside the cell. However, a knowledge of the unit cell and atomic coordinates alone is often insufficient to give a revealing picture of what the structure looks like in 3D. The latter is obtained only by considering a larger part of the structure, comprising perhaps several unit cells and by considering the arrangement of atoms relative to each other, their coordination numbers, interatomic distances, types of bonding, etc. It then becomes possible to find alternative ways of visualising structures and also to compare and contrast different types of structure.

Two of the most useful ways of describing structures are based on close packing and space‐filling polyhedra. Neither can be applied to all types of structure and both have their limitations. They do, however, provide greater insight into crystal chemistry than is obtained using unit cells and their contents alone. It can be very useful to make your own crystal structure models, either from coloured spheres or polyhedra, and tips for constructing models are given in Appendix B. In addition, all the structures described in this book, and many more, are available to view in the crystal viewer software package which can be downloaded, free of charge, from the Companion Wiley resource site. Using this, structures can be expanded, rotated, colours changed and key structural features highlighted by hiding selected atoms or polyhedra.