Sulfur chemisorption on Ni(111): The clock structure of the (5 sqrt 3 x 2)S phase.

From an interplay between x-ray diffraction and scanning tunneling microscopy the atomic structure of the (5\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}2)S reconstruction on Ni(111) has been revealed. It consists of four squarelike tetramers of Ni atoms, rotated in pairs \ensuremath{\sim}14\ifmmode^\circ\else\textdegree\fi{} clockwise and counterclockwise, onto which eight fourfold-coordinated S atoms are chemisorbed. The results have a significant bearing on the understanding of the detailed structure of postulated pseudo-(100) reconstructions of fcc(111) surfaces and support a general principle of increased adsorbate coordination as being decisive for the reconstruction.