Structure of Ultrathin CeO2 Films on Pt(111) by Polarization-Dependent X-ray Absorption Fine Structure

We present a study of the structure of cerium oxide ultrathin films supported on Pt(111), focused on the evolution of the epitaxial strain in films of different thickness. The stoichiometry and oxidation state of the films are determined by X-ray photoemission spectroscopy, and the surface structure, measured by low-energy electron diffraction, is compared with the results obtained by the analysis of X-ray absorption fine structure measurements at the Ce L-3 edge, exploiting the polarization dependence of the cross section to probe the in-plane and the out-of-plane atomic correlations. The obtained results allow to establish the epitaxial relation between the cerium oxide film and the Pt substrate and give an accurate evaluation of the cerium oxide layer structure. The 2 ML films have a fluorite structure which is compressed in the (111) plane. The measured compression is compatible with the assumption of a coincidence lattice between overlayer and substrate, in which three CeO2 surface unit cells match four Pt unit cells. The films' three-dimensional structure is compared with the one expected assuming the bulk-phase elastic constants. The strain is released when the film thickness is increased to 10 ML, and the lattice parameters assume the bulk values.