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 12 to 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]

Surface Diffusion of Compact and Other Clusters: Irx on Ir(111). S.C.Wang, U.Kürpick, G.Ehrlich: Physical Review Letters, 1998, 81[22], 4923-6

Figure 12

Diffusivity of Ir Atoms on the (111) Ir Surface

Figure 13

Diffusivity of 7-Atom Ir Groups on the (111) Ir Surface

Figure 14

Diffusivity of 18-Atom Ir Groups on the (111) Ir Surface

Figure 15

Diffusivity of 19-Atom Ir Groups on the (111) Ir Surface