On the prediction of order in the σ phases on the basis of a sphere-packing model

A theory for the order in σ phases based on the sphere-packing model of Wilson & Spooner [Acta Cryst. (1973), A29, 342-352] has been examined using a mathematical analysis. The analysis suggests that the prediction of order in these phases based on lattice-constant variations associated with variations in atomic diameter should be treated with caution, particularly where the results are at variance with experimentally determined results.