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

Abstract

Au-enriched (110) surfaces with Au coverages θ up to 1 monolayer (ML) were prepared by heating a Ni-0.8%Au crystal at temperatures T > 400° C. Low energy electron diffraction (LEED) indicated the following domains at T ≈ 20°C: 5× 1 (prominent for θ ≈ 0.7 ML), c(2 × 2) (0.7−0.9 ML). 7 × 7 (0.8−1.0 ML). The 7 × 7 domain was shown to contain c(2 × 4) subunits. The observations are shown to be consistent with a nearly close-packed 7 × 7 overlayer structure in which each c(2 × 4) subunit is associated with a hexagonal formation of six Au atoms, with a seventh Au atom at the center. Ni(210)-1 × 1 facets and Au(321)-3 × 1 facets due to Au crystallites were present on θ ≈ 1 ML surfaces. The effect of increasing T on θ and on LEED intensities was observed for θ ≈ 1 ML surfaces. θ was found to decrease by 15% on increasing T from 300 to 800°C. A first-order 7 × 7 → 2 × 7 phase transition was observed at T = 340 ± 10°C. Specific nearly close-packed overlayer models with AuAu equilibrium interatomic distance 2.7 Å were proposed for both the 7 × 7 and 2 × 7 surfaces. The models were shown to explain the observed LEED patterns qualitatively. Other LEED observations were interpreted tentatively to indicate a transition to a disordered overlayer structure for T > 550°C, disappearance of facets for T > 600°C, and one-dimensional melting of Au overlayers for T ≈ 800°C.