Importance of the atom-pair bond in metallic alloying

Rajasekharan and Girgis reported that binary systems with intermetallic compounds of a particular crystal structure form a straight line on a map using Miedema parameters. In this paper, the universality of that observation is examined. Observations from a study of 143 binary systems that crystallize in six different crystal structures at AB3 composition are discussed. Prediction of concomitant and mutually exclusive structure types in binary metallic phase diagrams, and of phase transitions among different structure types, has been demonstrated. This behavior is unexpected because Miedema parameters are isotropic in nature and structural energies are generally assumed to be small. We argue in this paper that each point on the map stands for the energy of an unlike atom-pair (A-B) bond, with the bond energy remaining nearly the same at all compositions in the phase diagram. This argument is confirmed by comparing the nearest-neighbour (A-B) bond lengths for the compounds of the structure types CaCu5 and CsCl, when concomitant with MgCu2 structure type. This fact leads to an important conclusion that one can define a bond energy for the metallic unlike-atom-pair bond as is usually done for a conventional chemical bond.