OBSERVATION OF PHASE-TRANSITIONS AT THE (100) SURFACE OF AL-3 AT-PERCENT AG

In the bulk binary alloy Al-3 at.% Ag, Ag2Al precipitates are formed below 410 degrees C which are reversibly dissolved at elevated temperatures. We have followed this phase transition at a (100) surface as a function of temperature by monitoring the bandwidth of the Ag 4d 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 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.