Thermodynamic Phase Diagram of Selenium Adsorbed on the NICKEL(100) Surface.

The detailed phase diagram for submonolayers of selenium adsorbed on the Ni(100) surface has been determined for the first time using the experimental methods of Auger Electron Spectroscopy, reflection electron diffraction and thermal desorption spectroscopy. Three phases are observed: (1) an order p(2 x 2) phase which forms for coverages near one-quarter monolayer and temperatures below about 500K; (2) an ordered c(2 x 2) phase which forms for coverages between one-quarter and one-half monolayer; (3) a disordered phase. The characteristics of the boundaries between these phases have been examined and, although first -order transitions cannot be unambiguously ruled out, these boundaries are most consistent with continuous (second -order) transitions. The phase boundaries meet at a multicritical point near a coverage of one-quarter monolayer and a temperature of about 500K. These results are analyzed in the context of current theories of phase transitions and universality. It is concluded that the Se/Ni(100) phase diagram may be a physical realization of a particular two-dimensional model known as the Ashkin-Teller model. The implications of this result for future studies are described.