By using field ion microscopy, direct observations were made of the diffusion of Rh and Ir tetramers on the Ir(001) surface. On the basis of the probabilities of observing given atomic configurations during diffusion, various diffusion mechanisms for Rh and Ir tetramers were compared. It was found that whether or not the square shape was more stable, than the linear shape, was the determining factor in the diffusion mechanism. The dimer-shearing mechanism on metal (001) surfaces was energetically favored for systems where the most stable structure was square. For both Ir and Rh tetramers on the Ir(001) surface, the most stable cluster structure was linear, but the energy differences between the linear structure and other structures for Rh tetramers were smaller than those for Ir tetramers. Diffusion mechanisms which were coupled with structural transformations were observed for Rh tetramers on the Ir(001) surface.

Structure and Diffusion Mechanism of Ir and Rh Tetramers on Ir (001) Surfaces. T.Y.Fu, T.T.Tsong: Surface Science, 2001, 482-485[2], 1249-54