The electronegativity parameter for transition metals: Heat of formation and charge transfer in alloys

It is demonstrated that a cellular model gives an excellent account of the heat of formation of binary alloys of 27 transition metals. The energy effects are described by two terms, the first representing the difference in electronegativity between the two types of atoms in an alloy and the second term reflecting the discontinuity in the density of electrons at the boundary between dissimilar Wigner-Seitz atomic cells. The electronegativity is expressed in a scale which closely resembles that of the experimental work functions of pure metallic elements. The use of this scale also gives a clear insight into the heat effects which accompany the alloying of transition metals with non-transition metals. If the latter have p-electrons a negative term adds up to the heat of formation which apparently does not depend on the particular metals considered. As a result, simple rules for the alloying behaviour of transition metals are formulated which have an accuracy between 96 and 100%; the electronegativity concept is shown to apply also for hydrogen in metals. The relation between the charge transferred per atom, ΔZa, and the difference in electronegativity, Δφ∗, is discussed quantitatively. As an example, the result for solid solutions of two transition metals is ΔZa = 1.2 (1 − ca)Δφ∗, where ca is the concentration of metal A. Our analysis supports the idea that the work function of a metal is, in principle, proportional to the chemical potential for electrons in Wigner-Seitz atomic cells; the proportionality factor is found to be about 1·4.