The structure of sulphur adsorption phases on Ni(111) studied by X-ray standing wavefield absorption

The technique of normal-incidence X-ray standing wavefield (NIXSW) absorption has been applied to an investigation of several structural phases of S on Ni(111), specifically the (2 x 2), (root 3 x root 3)R30 degrees and (5 root 3 x 2)rect. phases, with a view in particular to clarifying recent controversy over the structure of the last of these. Absolute adsorbate site determination has been effected through real-space triangulation using both(lll) and (<(1)over bar 11>) Bragg reflections. For the (2 x 2) phase the results are consistent with a simple overlayer structure with S atoms occupying the ''foc'' hollow sites (directly above third-layer Ni atoms), in agreement with earlier determinations by LEED, ion scattering and SEXAFS. The same local site is found to be occupied in the (root 3 x root 3)R30 degrees phase, although the NISXW data indicate poorer local order. For the (5 root 3 x 2)rect. phase, a simple undistorted missing-row model is found to be inconsistent with the data, as is any ideal ''coincidence lattice'' structure, including a simple undistorted pseudo-(100) surface reconstruction. A modification of this latter model, in which some S atoms penetrate the pseudo-(100) layer to bond to the underlying (111) substrate does, however, contain the main ingredients needed to fit the data. By using not only (111) and (<(1)over bar 11>) NIXSW but also (1(1) over bar1$) and (11(1) over bar) absorption profiles, recorded from a near single-domain structure formed on a miscut crystal surface, specific information on some of the structural parameters of this modified reconstruction model is obtained and compared with complementary data from other published studies.