Abstract
A combination of scaled particle theory for fluids and a simple cell model for crystalline solids is used to study the freezing behaviour of a binary hard-sphere system. As the two kinds of sphere become increasing dissimilar, we find that the phase diagram changes from a spindle type to an azeotropic type, and then to a eutectic type. This progression is the same as was first predicted using density functional theory and verified later using computer simulations. Due to the stringency of the cell boundaries, the use of the cell model exaggerates the effects of differences in the sizes of the spheres even more than the use of density functional theory. However, the simplicity of the cell model permits the evaluation of more extreme cases than either of the other two approaches.