Bombardment-induced compositional change with alloys, oxides, oxysalts, and halides. II. The role of segregation

Abstract

Compositional changes in bombarded alloys are now recognized to arise mainly from bombardment-induced segregation, "BIS" and to only a limited extent from preferential sputtering, i.e. from differences in mass, chemical binding, or volatility. The significant relation is αA(2) αB(2) = (αA(2′) αB(2′))Kb, where α is the atom fraction, A and B are the components of the alloy, 2 designates atom layer one, 2′ designates atom layer two and Kb is the BIS ratio. A very similar relation holds for equilibrium segregation except that Kb is replaced with Keq, equivalent to exp (ΔGseg/kT). It is shown that Kb and Keq are always in the same sense except for NiPt, that Kb lies in the interval 1.25 ≤ Kb ≤ 5.4 for 14 out of 16 different systems and that for ambient temperature the inequality Kb ≪ Keq holds. Most aspects of the relation between Kb and Keq can be understood if BIS is caused by chemically guided final steps in some ballistic trajectories. Such a mechanism also accounts for what is otherwise a paradox, namely that the driving force for segregation is so small, ranging from 0.06 eV for CuPd to 0.52 eV for AuNi. The latter values are similar to heats of mixing, proposed by Cheng et al. to govern ion-beam mixing and to differences of heats of formation, proposed by the present author to govern chemical changes in oxides and oxysalts. © 1989.