Clustering and ordering in solid solutions

Theoretical models of order-disorder and clustering reactions in solid solutions are briefly reviewed. The simplest model is that based on the point-cluster expansion, or single-site average, which yields a generalized multicomponent Bragg-Williams, model. The contribution to the free energy which is quadratic in the configuration variables (harmonic portion) governs the behavior of high-temperature concentration fluctuations and of incipient Brillouin zone-center (clustering) and zone-boundary (ordering) instabilities. Anharmonic contributions determine the equilibrium structures. The description of states of partial order from knowledge of short-range-order intensity is briefly discussed.