Calculated activation barriers and pre-exponential factors showed that monovacancy diffusion was an important contributor to mass transport on the (100) surfaces, and competed well with adatom diffusion. The contribution of lattice vibrations to the temperature dependence of the diffusion coefficient was incorporated via activation functions which were calculated in the harmonic approximation; using transition-state theory and many-body interaction potentials from the embedded-atom method. The activation barriers on all 3 of the metal surfaces were found to increase upon including lattice thermal expansion. The results showed that the vibrational entropy contributions, and hence the pre-factors, depended upon particular features of the local density of states. It was found that, at temperatures above 200K, the pre-factors became almost constant. At temperatures below 200K, they decreased with decreasing temperature and vanished at 0K; due to the use of the quantum-mechanical partition function. Although the quantitative details depended upon the details of the interaction potential, it was found that the diffusion coefficients for monovacancies were comparable to those for adatoms; if not larger. The role played by vacancies therefore could not be ignored.

Monovacancy Diffusion on Ag(100), Cu(100) and Ni(100): Prefactors and Activation Barriers. U.Kürpick, T.S.Rahman: Physical Review B, 1999, 59[16], 11014-9