Compounds of the Type (A, B)X

Compounds such as gallium aluminum arsenide are of interest for two reasons. First, they have practical use, so that benefit comes from understanding their properties and behavior; but second, they can be regarded as mixtures of just two components, GaAs and AlAs, and so serve as a fresh set of examples of the ideas in Chapter 14. A difficulty in that chapter arises from the fact that at some points we need to imagine a mechanical continuum, while at other points we need to imagine particles traveling independently. In this chapter we need to do the same two things but the conflict in our concepts is not as acute; we can use eqn. (14.9) with more confidence and escape from the sense of an internal contradiction. The basis for the discussion is the idea of perfect stoichiometry; in a compound of type (A, B)X it is assumed that although the abundance-ratio of A to B is variable, the ratio of (A + B) to X is always exactly 1. Departures from stoichiometry are, of course, of great importance but constitute a later topic.