Long-Range Displacive Reconstruction of Au(110) Triggered by Low Coverage of Sulfur

We propose a new model for the c(4 X 2) phase of sulfur adsorbed on Au(110). This is a reconstruction achieved by short-range rearrangements of Au atoms that create a pseudo-4-fold-hollow (p4fh) site for adsorbed sulfur. The model is based partly upon the agreement between experimental STM images and those predicted from DFT, both within c(4 X 2) domains and at a boundary between two domains. It is also based on the stability of this structure in DFT, where it is not only favored over the chemisorbed phase at its ideal coverage of 0.25 ML, but also at lower coverage (at T = 0 K). This is compatible with the fact that in experiments, it coexists with 0.06 +/- 0.03 ML of sulfur chemisorbed on the (1 x 2) surface. The relative stability of the c(4 x 2) phase at 0.25 ML has been verified for a variety of functionals in DFT. In the chemisorbed phase, sulfur adsorbs at a pseudo-3-fold-hollow (p3fh) site near the tops of rows in the (1 X 2) reconstruction. This is similar to the fcc site on an extended (111) surface. Sulfur causes a slight separation between the two topmost Au atoms, which is apparent both in STM images and in DFT-optimized structures. The second-most stable site is also a p3fli site, similar to an hcp site. DFT is used to construct a simple lattice gas model based on pairs of excluded sites. The set of excluded sites is in good qualitative agreement with our STM data. From DFT, the diffusion barrier of a sulfur atom is 0.61 eV parallel to the Au row, and 0.78 eV perpendicular to the Au row. For the two components of the perpendicular diffusion path, that is, crossing a trough and hopping over a row, the former is considerably more difficult than the latter.