The modified quasi-chemical model: Part IV. Two-sublattice quadruplet approximation

The modified quasi-chemical model is further extended, in the quadruplet approximation, to treat, simultaneously, first-nearest-neighbor (FNN) and second-nearest-neighbor (SNN) short-range ordering (SRO) in solutions with two sublattices. When one sublattice is occupied by only one species, or is empty, the model reduces to the modified quasi-chemical model for one sublattice in the pair approximation. The coordination numbers and the ratio of the number of sites on the two sublattices are permitted to vary with composition, thereby making the model ideally suited to the treatment of liquid solutions such as molten salts. The model also applies to solid solutions if the number of sites and coordination numbers are held constant and may be combined with the compound-energy formalism to treat SRO in a wide variety of types of solutions. A significant computational simplification is achieved by formally treating the quadruplets as the "components" of the solution.