Modelling and measurement of A1 interlayer diffusion in Pd(100): a low-energy ion scattering study

The Pd(001)-(2×2)p4g-A1 surface consists of 0.5 monolayer of A1 diffusing into the second layer during annealing and causing a p4g clock reconstruction. This is an interesting bimetallic alloy due to the ability to shift the A1 cyclically and reproducibly from the second layer to the first and then back again. This is achieved by lifting the A1 from the second layer by the absorption of O₂ at room temperature and then removing the oxygen with hydrogen at 200 °C. If the surface is again heated, the A1 diffuses back into the second layer and again produces the clock reconstruction. This cyclic process has been found to be repeatable continuously, and scanning tunneling microscope studies have revealed that the diffusion takes place midterrace rather than at a terrace edge. A model has been derived to calculate the activation energy for this process using Fick's second law with a special boundary condition on the second atomic layer. Furthermore, using low-energy ion scattering, the activation energy for the diffusion of A1 from the first to the second atomic layer has been measured to be 0.41±0.02 eV. This value suggests that Pd atoms are segregating through the Al to the surface via the exchange process. Measurements at higher temperatures have revealed that the activation barrier for diffusion between the second and third layers is 2.0±0.4 eV, which suggests that the diffusion of the A1 into the bulk is via the vacancy mechanism