The interaction of C with Ni(100) has been studied by scanning tunneling microscopy (STM). The C atoms, imaged as ≊0.3 Å 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 Å lateral clockwise-counterclockwise displacement of the topmost Ni atoms. A simple model for the driving force for this ‘‘clock’’ reconstruction is proposed.

• Received 19 August 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.4350