CAN OXYGEN MODIFY STEP ARRANGEMENTS - STM AND LEED INVESTIGATIONS ON NI(771)

Clean Ni(771), a vicinal Ni(110) surface, stabilizes in bulk truncated form with single terrace widths and regular (111) steps of single height parallel to the close-packed rows as revealed by scanning tunneling microscopy and low energy electron diffraction. Ni(771) therefore offers the interesting possibility to study the role of steps on the gradual formation of the oxygen-driven (2 x 1) reconstruction of the missing row (MR) type as a function of oxygen coverage. After oxygen exposures of 20 L the surface exhibits a perfect (2 x 1) MR configuration with the terraces and steps modified to double width and height, respectively. Thus the missing/ added row mechanism for the formation of the MR-reconstruction is directly visualized. At low oxygen exposures isolated linear Ni/O chains are formed that already extend over twice the initial terrace width. At oxygen coverages of about 60% of that of the (2 x 1) phase a peculiar (6 x 1) superstructure is observed; its strucural units are separated by 3a1 (a1 = a/square-root 2) and comprise of three linear metal chains with distances of 2a1 and a1, respectively.