Step-modified phase diagram of chemisorbed oxygen on nickel

We have studied the effect of an extended array of steps on the two-dimensional phase behavior of chemisorbed oxygen overlayers on a vicinal nickel surface using low energy electron diffraction (LEED), Auger electron spectroscopy, and scanning tunneling microscopy. Phase behavior of oxygen on the vicinal Ni(9 7 7) surface was examined and compared with that for oxygen adsorbed on the flat Ni(1 1 1) surface. There are two significant differences in the phase diagrams for these two surfaces. On Ni(1 1 1) at θ=0.25 ML, oxygen forms a p(2×2) structure that disorders to a lattice gas at 440 K and remains disordered until it is ultimately dissolved into the bulk above 500 K. Surface defects, such as the steps on Ni(9 7 7), substantially modify this phase progression. On Ni(9 7 7), the p(2×2) phase still disorders at 440 K, but a second ordered phase, which can be designated as Ni[8(1 1 1)×(1 0 0)]–2(1d)-O in microfacet notation, exists between room temperature and above 500 K when the oxygen is finally incorporated into the bulk. This adsorbate phase is step-stabilized and can be generated by dosing the surface with a small amount of oxygen or as a result of partial dissolution of oxygen from the higher coverage p(2×2) phase. Moreover, anisotropic disordering effects are evident due to the presence of the steps as indicated by the increasingly oblate shape of diffraction spots as the p(2×2) disorders. The process of oxygen dissolution is also qualitatively altered by the presence of regular steps.