Ring clusters in transition-metal–silicon surface structures

We have determined the atomic structure of the Si(111)-(√7× √7)Co surface using scanning tunneling microscopy and ion scattering. The unit cell contains one surface substitutional cobalt atom centered under a six silicon adatom cluster. High-temperature annealing produces a low density lattice gas of these ringlike clusters, giving rise to an ‘‘impurity stabilized 1×1’’ structure. They occur for several other transition metals, suggesting a stable, universal, silicide related structure.