The classification of crystal structures

We have now completed our reviews of the four principal types of interatomic force responsible for the cohesion of solids, and have illustrated by examples some characteristic properties of simple crystal structures in which these different types of force operate. These properties, and also some additional physical properties which we shall not discuss in detail, are summarized in table 6.03. 

In the first edition of this book these four types of bond were used as the basis for a system of classifying crystal structures. It has now become abundantly clear, however, as we have repeatedly emphasized, that the distinction between these types of force is less real than was at one time imagined, and is by no means absolute in many structures binding of an 

Structural Non-directed, giving structures of high co-ordination Spatially directed and numerically limited, giving structures of low co-ordination and low density Non-directed, giving structures of very high co-ordination and high density Formally analogous to metallic bond 

Mechanical Strong, giving hard crystals Strong, giving hard crystals Variable strength. Gliding common Weak, giving soft crystals 

Thermal Fairly high M.P. Low coefficient of expansion. Ions in melt High m.p. Low coefficient of expansion. Molecules in melt Variable M.P. Long liquid interval Low M.P. Large coefficient of expansion 

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