It was noted that the Pd(001)-(2×2)p4g-Al surface consisted of 0.5 monolayer of Al diffusing into the second layer during annealing, and causing a p4g clock reconstruction. Its effect was to shift the Al cyclically and reproducibly from the second layer to the first one, and back. This involved lifting the Al from the second layer by the absorption of O at room temperature, and then removing the O with H at 200C. If the surface was again heated, the Al diffused back into the second layer and again produced the clock reconstruction. This cyclic process was continuously repeatable, and scanning tunneling microscopy revealed that the diffusion took place mid-terrace rather than at a terrace edge. A model was derived, in order to calculate the activation energy for this process, by using Fick’s second law; with a special boundary condition imposed upon the second atomic layer. By using low-energy ion scattering, the activation energy for Al diffusion from the first to the second atomic layer was deduced to be 0.41eV. This value suggested that Pd atoms segregated through the Al, to the surface, via an exchange process. Measurements at higher temperatures revealed that the activation barrier to diffusion between the second and third layers was 2.0eV. This suggested that diffusion of Al into the bulk occurred via the vacancy mechanism.

Modeling and Measurement of Al Interlayer Diffusion in Pd(100) - a Low-Energy Ion Scattering Study. M.J.Gladys, F.Samavat, B.V.King, D.J.O’Connor: Physical Review B, 2004, 69[16], 165418 (6pp)