Three-dimensional clusters of metallic Ce are deposited on Pt(111) surfaces by thermal evaporation in ultrahigh vacuum. Different reactions occur upon heating of the sample in ultrahigh vacuum and during the exposure of oxygen that lead to distinct well-ordered surface phases. Their geometric structures are determined by scanning tunneling microscopy (STM). A two-dimensional ordered surface alloy of ${\mathrm{Pt}}_5\mathrm{Ce}$ is obtained by annealing at 1000 K. It forms two different incommensurate overlayers on Pt(111) with long-distance spacings in the moiré patterns (13.7 and 14.8 Å). Hexagonally shaped two-dimensional islands are formed upon heating the Pt(111) single crystal with a submonolayer coverage of Ce at 900 K. They are localized at step edges. STM reveals a local $2\times 2$ structure with respect to Pt(111). It is assigned to a precursor state of alloy formation. Ordered ${\mathrm{CeO}}_2$ phases result from annealing the alloy at 1000 K in oxygen. The surface structure is consistent with oxygen-terminated fluorite-type ${\mathrm{CeO}}_2\mathrm{}\left(111\right).$ During decomposition of ${\mathrm{CeO}}_2$ at 900 K, an oxide structure is identified which is attributed to an ultrathin ordered phase of surface-${\mathrm{Ce}}_2{\mathrm{O}}_3.$

• Received 9 October 2001

DOI:https://doi.org/10.1103/PhysRevB.65.235404