THE BOND-ENERGY MODEL FOR ORDERING IN A PHASE WITH SITES OF DIFFERENT COORDINATION NUMBERS

The bond energy model (BEM) is applied to the case of ordering where the sites on different sublattices have different coordination numbers. A perceived problem in previous attempts to apply the BEM to this situation, which led to the introduction of the compound energy model (CEM), is shown to have arisen from neglecting to recognize the greater importance of the site exchange energies, of which there are two for each pair of sublattices, rather than the pair exchange energy, in determining the properties of the solution. The BEM has the advantage over the CEM for multiple sublattice solutions when analytical calculations are made in which the assumption of random mixing on the sublattices is relaxed or when it is desired to carry out Monte Carlo simulations.