The structure of the Pd(001)-(2 x 2)p4g-Al surface was investigated using low-energy ion scattering, low-energy electron diffraction, and three-dimensional classical scattering simulations. The thermal treatment of the Al films (θAl>~0.5ML) triggered the Al diffusion and reaction to form a clock rotated (001) Pd layer above an ordered c(2 x 2) Al-Pd underlayer, with a stoichiometry of the top two layers independent of the initial Al coverage. By using a reliability R-factor analysis to compare the experimental and simulated azimuthal scans, the lateral clockwise-counterclockwise displacement of the surface Pd atoms was determined to be Δx=0.5Å. The driving force for this clock reconstruction was proposed to be the Al-induced interfacial strain. The oxygen-induced lifting of the Pd(001)-(2 x 2)p4g-Al reconstruction was also studied. Adsorption of oxygen on the (2 x 2)p4g surface at room temperature induced Al segregation and lifted the reconstruction to yield the (1 x 1) phase. Oxygen removal from the (1 x 1) surface by higher temperature annealing (∼900K) was accompanied by depletion of Al from the surface, recovering the reconstruction. The mechanism of a reversible conversion, (2 x 2)p4g→(1 x 1) by O adsorption and (1 x 1)→(2 x 2)p4g by anneal, was considered.

Aluminum-Induced Surface Clock Reconstruction of Pd(001) and the Effect of Oxygen Adsorption. Y.G.Shen, J.Yao, D.J.O’Connor, B.V.King, R.J.MacDonald: Physical Review B, 1997, 56, 9894-901