Step-modified phase diagram of chemisorbed oxygen on nickel

We have studied the effect of an extended array of steps on the two-dimensi onal phase behavior of chemisorbed oxygen overlayers on a vicinal nickel su rface using low energy electron diffraction (LEED), Auger electron spectros copy, and scanning tunneling microscopy. Phase behavior of oxygen on the vi cinal Ni(9 7 7) surface was examined and compared with that for oxygen adso rbed on the flat Ni(111) surface. There are two significant differences in the phase diagrams for these two surfaces. On Ni(111) at theta = 0.25 ML, o xygen forms a p(2 x 2) structure that disorders to a lattice gas at 440 K a nd 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 modif y this phase progression. On Ni(9 7 7), the p(2 x 2) phase still disorders at 440 K, but a second ordered phase, which can be designated as Ni[8(111) x (100)]-2(1d)-O in microfacet notation, exists between room temperature an d above 500 K when the oxygen is finally incorporated into the bulk. This a dsorbate phase is step-stabilized and can be generated by dosing the surfac e with a small amount of oxygen or as a result of partial dissolution of ox ygen from the higher coverage p(2 x 2) phase. Moreover, anisotropic disorde ring effects are evident due to the presence of the steps as indicated by t he increasingly oblate shape of diffraction spots as the P(2 x 2) disorders . The process of oxygen dissolution is also qualitatively altered by the pr esence of regular steps. (C) 2001 Elsevier Science B.V. All rights reserved .