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The C14(MgZn2), C15(1VfgCu2) and Cs6(MgNi2) crystal structures frequently occur at the composition AB z in alloy systems for which the ratio of the atomic radii of the components is approximately 1-2.In certain ternary magnesium alloys interesting structural changes from one type to another occur as other metals are substituted for zinc and copper in MgZn~ and MgCu~ respectively. In an attempt to understand these changes, and the factors determining which of the closely related structures is assumed in a given case, an examination has been made of the interatomic distances in phases for which data are available. A parameter S which measures the deviation of particular interatomic distances (dAA or dBB) from the corresponding distances in the pure components (d~ or dB) is plotted against W = dA/dB. For series of phases with common A or B components, the relationship between W and S is approximately linear. If W lies close to the ideal value for the formation of a Laves phase, S is small, and hexagonal C14 structures occur. As W deviates from the ideal value, S increases numerically, and cubic C15 structures occur. This suggests that, for a given series of phases with common A or B components, the C14 structure is not in general stable above or below a critical W value. When the B component is a transitional metal, more complex graphs are obtained; there is evidence in these cases for relatively strong A-B interactions. Assuming that increasing S favours the C]5 structure, and taking into accotmt the effect of changing Fermi energy, an interpretation of the structural changes observed in ternary magnesium alloys is possible. The considerations outlined in this paper should help in the prediction of Laves phases in alloy systems as yet not investigated.

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