Bombardment-induced compositional change with alloys, oxides, oxysalts and halides III. The role of chemical driving forces

Abstract

Recent experiments suggest that weak chemical driving forces play a role in bombarded targets even when, for reasons of extreme energy disparity, it might be difficult to understand why this should be so. With bombardment-induced segregation the driving force, assuming the process to resemble equilibrium segregation, is 0.06–0.52 eV atom −1. We suggest that the segregation is the result of a certain fraction of the ballistic trajectories terminating near the target surface being followed by one or more low energy, chemically guided steps. With bombardment-induced mixing the driving force, according to Cheng et al., is typically 1.3 eV atom −1 or less. We propose a very similar model to that of segregation, namely ballistic trajectories with terminal steps which are chemically guided. Finally, with bombardment-induced decomposition, as when oxides lose oxygen or sulfates lose sulfur and oxyten, there appears to be a role for what are effectively phase changes in which the system appears to tolerate an energy increase of up to 0.7–0.8 eV atom −1 but not a larger amount. The changes, which resemble those analyzed by Fecht and Johnson, are aided by a number of factors, including amorphization, point-defect accumulation, volatility, diffusional transport and segregation. We tend to avoid in all cases a thermal spike type of argument, if only because the inequality (driving force) ⪆ kT must be obeyed.