Thin layers of U on polycrystalline Pd and Pd(100) were prepared by magnetron sputter deposition at room temperature. Annealing-induced surface rearrangements and bulk diffusion were then studied by means of Auger electron spectroscopy, X-ray photo-electron spectroscopy and ultra-violet photo-electron spectroscopy. It was found that, at low coverages, the 5f electrons of U were localized, and reactivity was suppressed up to the point where CO dissociation became an activated process. The U remained on the top surface, where it was atomically dispersed. At higher coverages, the U became reactive; as shown by spontaneous U-Pd interdiffusion and by the dissociation of CO. It was noted that O2 always reacted with the surface (oxidizing U to UO2), and that annealing always triggered the diffusion of U into Pd. Two types of diffusion were observed: at temperatures below 250C it resulted in U-Pd surface compound formation whereas, at temperatures above 300C, it involved the bulk diffusion of U.

T.Gouder, C.A.Colmenares: Journal of Alloys and Compounds, 1994, 213-214, 500-2