Quantitative measurements were made of the diffusivity of larger Ir clusters on (111) Ir surfaces in order to explore the mechanisms of motion. In the case of non-compact Ir18, migration occurred via changes in the overall shape as atoms diffused along cluster edges. The pre-factor was comparable to the value for single atom motion. In the case of Ir19 and Ir7, which were clusters that existed as compact closed shells, diffusion via changes in overall shape was shown to be unlikely. Migration occurred via an entirely different mechanism. The pre-factors here were some 3 orders of magnitude higher than that for Ir18. The data for single atoms and various groups (figures 14 and 15) could be described by:

Ir     D (cm2/s) = 3.8 x 10-4 exp[-0.29(eV)/kT]

Ir7     D (cm2/s) = 1.4 exp[-1.49(eV)/kT]

Ir18     D (cm2/s) = 7.8 x 10-4 exp[-1.63(eV)/kT]

Ir19     D (cm2/s) = 13 exp[-2.54(eV)/kT]

S.C.Wang, U.Kürpick, G.Ehrlich: Physical Review Letters, 1998, 81[22], 4923-6

Figure 14

Diffusivity of Ir Atoms on the (111) Ir Surface