Band theory for order-disorder phase transformations: CuAu

We present a band theory for the internal energy of binary alloys which undergo order-disorder transformations. The general method involves the self-consistent solution of a tight-binding Hartree Hamiltonian in the presence of both long- and short-range-order correlations through the use of an extended cluster-Bethe-lattice method. A complete scheme for the derivation of the alloy thermodynamics is obtained by coupling the electronic theory to the cluster-variation combinatorial theory for the alloy configurational entropy. The theory is applied to the study of the CuAu order-disorder transition and comparisons are made to existing data and Ising models.