Aluminum-induced surface clock reconstruction of Pd(001) and the effect of oxygen adsorption

The structure of the $\mathrm{Pd}\mathrm{}\left(001\right)-(2\mathrm{}\times 2)p4g-\mathrm{Al}$ surface has been investigated using low-energy ion scattering, low-energy electron diffraction, and three-dimensional classical scattering simulations. The thermal treatment of the Al films $({\theta }_{\mathrm{Al}}>~0.5\mathrm{M}\mathrm{L})$ triggered the Al diffusion and reaction to form a clock rotated (001) Pd layer above an ordered $c(2\mathrm{}\times 2)$ 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 $\Delta x=0.5\mathrm{}\pm 0.1$ Å. The driving force for this clock reconstruction is proposed to be the Al-induced interfacial strain. We have also studied the oxygen-induced lifting of the $\mathrm{Pd}\mathrm{}\left(001\right)-(2\mathrm{}\times 2)p4g-\mathrm{Al}$ reconstruction. Adsorption of oxygen on the $(2\mathrm{}\times 2)p4g$ surface at room temperature induces Al segregation and lifts the reconstruction to yield the (1×1) phase. Oxygen removal from the (1×1) surface by higher temperature annealing (∼900 K) was accompanied by depletion of Al from the surface, recovering the reconstruction. The mechanism of a reversible conversion, $(2\mathrm{}\times 2)p4g\to (1\mathrm{}\times 1)$ by O adsorption and $(1\mathrm{}\times 1)\to (2\mathrm{}\times 2)p4g$ by anneal, is discussed.