  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.
