Structure and diffusion mechanism of Ir and Rh tetramers on Ir(0 0 1) surfaces

Using the field ion microscope, we observe directly the diffusion behavior of Rh and Ir tetramers on the Ir(001) surface. From the probability of observing different atomic configurations during diffusion, different diffusion mechanisms of Rh and Ir tetramers are investigated and compared. Whether the square shape is more stable than the linear shape or not is the determining factor of the diffusion mechanism. The dimer shearing mechanism [phys. Rev. Lett. 76(1996) 4927] on metal (001) surfaces is energetically favored for systems that the most stable structure is square. For both Ir and Rh tetramers on the Ir(001) surface, the most stable cluster structure is linear shape, but the energy differences between the linear structure and other structures for Rh tetramers are smaller than those for Ir tetramers. The diffusion mechanisms coupled with structure transformation are observed for Rh tetramers on the Ir(001) surface. The effect of the different energy differences is also discussed.