Observation of Phase Transitions at the Surface of Al--3 at.{\%} Ag

Authors: M. ERBUDAK, M. HOCHSTRASSER, E. WETLI

Abstract: In the bulk binary alloy Al--3 at.{\%} A0Ag, Ag_{2}Al precipitates are formed below 690 K which are reversibly dissolved at elevated temperatures. Here we report on this phase transition observed at two different surfaces. At the (111) surface, Ag--rich clusters are found which possess hexagonal stucture, characteristic of Ag_{2}Al. At the (100) surface the cluster size was followed as a function of temperature by monitoring the bandwidth of the Ag 4{\it d} states in x--ray photoelectron spectroscopy. Since the bandwidth measures the coordination number of the emitting atoms, it directly reveals the short--range order of the Ag atoms at the surface. The measurements show that the dissolution of the Ag--rich clusters already starts at temperatures at least 100 K below the bulk transition, and the observed hysteresis behavior is indicative of a first--order phase transition at the surface.

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