Structure transformation of Ir clusters on Ir surfaces

A small Ir cluster can assume either a one-dimensional linear-chain structure or a two-dimensional island-like structure. We present a study of the energetics of the 1 D to 2 D structure transformation of three-atom Ir clusters on the Ir(111) and (001) surfaces. On the (111) plane, the temperature dependence of the ratio of the probabilities of observing a three-atom cluster in the 1 D and 2 D structures exhibits a simple linear Arrhenius behavior. The 2 D island structure is found to be more stable with the cluster binding energy lower by 0.098±0.004 eV. On the (001) plane, the 1 D chain structure is more stable with the cluster binding energy lower by 0.335±0.015 eV. From these energies, the relative pair interaction at three different bond lengths can be derived. The relative pair potential is found to be non-monotonic in distance dependence. We explain the (1×5) reconstruction of the Ir(001) surface as being caused by the large difference in the pair binding energy of the first and second nearest-neighbor bonds. In addition, we find a significant deviation from the simple linear Arrhenius behavior at low temperatures for the three-atom Ir cluster on the Ir (001) plane, indicating that the entropy factor is temperature dependent.