HYBRIDIZED NEARLY-FREE-ELECTRON TIGHT-BINDING-BOND APPROACH TO INTERATOMIC FORCES IN DISORDERED TRANSITION-METAL ALLOYS .1. THEORY

A hybridized nearly-free-electron tight-binding-bond approach to interatomic forces in disordered transition-metal alloys is presented. The s-electron contribution to the pair and volume forces is calculated in a pseudopotential perturbation expansion; the d-electron contribution, within a tight-binding-bond approach. The bond order is calculated analytically on a Bethe-lattice reference system. It is shown that the bond order depends strongly on the form of the d band in the alloy. As the electronic density of states changes from a common band to a split-band form, the pair forces in the alloy change from a set of additive pair potentials to nonadditive potentials with a strong preference for the formation of pairs of unlike atoms and short bond distances in the AB pairs. This is illustrated for a series of Ni-M alloys (with transition metal M = Y, Zr,..., or Pd) and as a function of alloy composition.