Abstract

The volumes of formation of AuCu 3 and Cu 2 Mg prototypes in all the binary systems in which the two structures exist have been calculated to show that the contraction experienced per mole is higher for the AuCu 3 structure. The reasons for such behaviour are outlined. The different systems considered have radius ratios varying from 1.078 to 1.372. One normally expects the Cu 2 Mg structure to be better packed at large values of the radius ratio. In the platinum-based rare-earth systems, it has been observed that the AuCu 3 structure is better packed than the Cu 2 Mg structure as inferred from two other criteria, namely the percentage contraction in elemental volume and the deviation from Vegard's law