Interaction of C with Ni(100): Atom-resolved studies of the ‘‘clock’’ reconstruction

The interaction of C with Ni(100) has been studied by scanning tunneling microscopy (STM). The C atoms, imaged as {}0.3 {} deep depressions consistent with theoretical predictions, are at low C coverages located in the fourfold hollow sites, causing a small radial displacement of the surrounding Ni atoms. Above a local critical coverage, C induces a disorder-order transition to a reconstruced phase, and the STM images reveal a coordinated 0.55 {} lateral clockwise-counterclockwise displacement of the topmost Ni atoms. A simple model for the driving force for this ``clock'' reconstruction is proposed.