The migration of W dimers on the W(110) and (211) surfaces was investigated by using a fourth-moment approximation to tight-binding theory. In the case of the (110) surface, it was found that the optimum diffusion mechanism was a concerted jump. That is, adatom pairs which hopped simultaneously. Linear displacements of a W dimer in <111> directions were energetically favored over orientation changes. In the case of the (211) surface, there were 2 different diffusion mechanisms. For a dimer in one channel along <111> directions, the mechanism was a concerted jump with a high activation energy. For adatoms which were located in adjacent channels, the dimers diffused mainly via individual adatom jumps. Such dimer diffusion was highly correlated. The present work suggested that the activation energies for W dimer migration were similar to those for monomer diffusion. This result was in good agreement with field ion microscopic observations. The diffusion mechanisms and diffusion anisotropy could be described in terms of a simple bond coordination model.

W.Xu, J.B.Adams: Surface Science, 1995, 339, 247-57