The modified quasi-chemical model: Part III. Two sublattices

The modified quasi-chemical model in the pair approximation for short-range ordering (SRO) in liquids is extended to solutions with two sublattices. S hort-range ordering of nearest-neighbor pairs is treated, and the effect of second-nearest-neighbor (SNN) interactions upon this ordering is taken int o account. The model also applies to solid solutions, if the number of latt ice sites and coordination numbers are held constant. It may be combined wi th the compound-energy formalism to treat a wide variety of solution types. A significant computational simplification is achieved by formally treatin g the nearest-neighbor pairs as the "components" of the solution. The model is applied to an evaluation/optimization of the phase diagram of the Li,Na ,K/F,Cl,SO4 system.