Charge transfer on the metallic atom-pair bond, and the crystal structures adopted by intermetallic compounds

It has been argued in our recent papers that the heat of formation of intermetallic compounds is mostly concentrated in the nearest neighbor unlike atom-pair bonds, and that the positive term in Miedema's equation is associated with charge transfer on the bond to maintain electroneutrality. In this paper, taking examples of some well populated crystal-structure types such as MgCu₂, AsNa₃, AuCu₃, MoSi₂ and SiCr₃ types, the effect of such charge transfer on the crystal structures adopted by intermetallic compounds is examined. It is shown that the correlation between the observed size changes of atoms on alloying and their electronegativity differences is supportive of the idea of charge transfer between atoms. It is argued that the electronegativity and valence differences need to be of the required magnitude and direction to alter, through charge transfer, the elemental radius ratios R_A/R_B to the internal radius ratios r_A/r_B allowed by the structure types. Since the size change of atoms on alloying is highly correlated to how different R_A/R_B is from the ideal radius ratio for a structure type, the lattice parameters of intermetallic compounds can be predicted with excellent accuracy knowing R_A/R_B. A practical application of the approach developed in our recent papers to superalloy design is presented.