Diffusion, Reaction and Structure of Complexions of Bismuth and lead at the Copper(100) Surface

Dominique Chatain 1 Jean-Philippe Monchoux 2 Paul Wynblatt 3
1Centre Interdisciplinaire de Nanoscience de Marseille - CNRS, Aix-Marseille University, Marseille
2Centre d'Elaboration de Matériaux et Etudes Structurales, CNRS, Toulouse
3Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA
We have studied a two-dimensional diffusion couple, formed when two adsorbed layers of pure Pb and Bi spread towards each other and meet, on a Cu(100) surface at 513 K. The adsorbed layers were fed by two bulk sources of pure Pb and pure Bi deposited on the copper substrate 3mm apart from each other.

The surface diffusion and composition of the diffusing layers of Pb and Bi on Cu (100) were investigated in situ in a scanning Auger microscope. Different surface structures of the Bi along the diffusing layer were observed in situ by STM. Like Pb, the diffusing Bi layer is made of a sequence of complexions which belong to the surface complexion diagram of Bi on Cu(100); they span from dense "on-top phases" close to the Bi source to "alloyed lattice gas phases" at low coverage far from the source. The coverage profiles of both components along the diffusing layers display steps at about one monolayer and half a monolayer. These compositional steps are related to coexistence of two different surface structures of the complexions. Whatever the coverage, Pb diffuses faster than Bi in the various complexions.

Interdiffusion/reaction displays different stages. When the partial monolayers of Pb and Bi meet, Pb diffusion slows down. The junction between the two components drifts towards the Bi source. Surprisingly, the drift of the junction line reverses direction towards the Pb source, when the two complete monolayers meet. Superimposed on this drift, the Pb and Bi layers undergo interdiffusion. Pb and Bi fully mix within the monolayer, whereas the bulk phases display only partial solubilty. The phenomena occuring during two-dimensional interdiffusion are related to the surface structures.








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