Low energy electron diffraction observations on Au-enriched Ni-0.8% Au Alloy (110) surfaces prepared by surface segregation.

Au-enriched (110) surfaces with Au coverages theta up to 1 monolayer (ML) were prepared by heating a Ni-0.8% Au crystal at temperatures T > 400C. Low energy electron diffraction (LEED) indicated the following domains at T~20C: 5x1 (prominent for theta ~ 0.7ML), c2x2 (0.7-0.9 ML), 7x7 (0.8-1.0ML). The 7x7 domain was shown to consist of ribbon-like 2x7 domains separated by out-of-phase [001] boundaries, where the 2x7 domains contain c2x4 subunits. Ni(210)-1x1 facets and Au(321)-3x1 facets due to Au crystallites were present on theta~1ML surfaces. The effect of increasing T on theta and on LEED intensities was observed for theta~1ML surfaces. Theta was found to decrease by 15% on increasing T from 300 to 800C. A first-order 7x7 -> 2x7 phase transition was observed at T = 340 +- 10C. The observation was shown to be consistent with an enhanced probability of occurrence of in-phase relative to out-of-phase [001] boundaries for higher T. Other LEED observations were interpreted tentatively to indicate a transition to a disordered overlayer structure for T > 550C, disappearance of facets for T>600C, and one-dimensional melting of Au overlayers for T~800C.