Precipitation and re-solution of impurities at the surface of indium on traversing the melting-point

In the course of preparing a clean indium surface for solid-versus-liquid studies, changes in the surface concentrations of sulphur and oxygen were observed by AES and XPS while the metal was heated and cooled through its melting-point. Both impurities disappeared on melting, and reappeared on solidification, over a very narrow temperature range; the disappearance and reappearance were to a certain extent reproducible. The effect was found to be similar in characteristics to that observed for the behaviour of carbon on a nickel surface by Shelton et al., and the same Bragg-Williams model is invoked to explain the sharpness of the impurity concentration changes with temperature. Although the maximum temperature reached by the indium was only 200°C, traces of platinum were also observed on the indium surface after melting, in both the AES and XPS spectra, probably as a result of solution from the platinum boat.