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Stabilization of the ceria ι-phase (Ce7O12) surface on Si(111)
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Image of FIG. 1.
FIG. 1.

(a) SPA-LEED pattern obtained at 59.3 eV electron energy after annealing in UHV (660 °C/30 min). The fundamental diffraction spots exhibit a triangular shape and three-fold symmetry. Also a ( )R19.1° superstructure with two mirror domains is visible. (b) Sketch of the diffraction pattern. The reflexes of the two mirror domains of the (  ×  )R19.1° superstructure are colored in red and blue.

Image of FIG. 2.
FIG. 2.

After annealing at 660 °C the cub-CeO2(111) fluorite surface unit cell (solid line) is replaced by the (  ×  )R19.1° superstructure (dashed line) due to formation of oxygen vacancies. This unit cell contains 7 cerium and 12 oxygen atoms and, therefore, it can be assigned to the ι-phase of ceria. The mirror domain is not shown for clarity.

Image of FIG. 3.
FIG. 3.

XP spectra of the Ce 3d region. The area underneath the peaks is normalized to one after a Shirley background is subtracted from all spectra. The plasma treated CeO2 spectrum (green) consists of three peaks in each 3d5∕2 and 3d3∕2 component. All cations have Ce4+ valence state. The reduced Ce2O3 spectrum (blue) consists of two peaks in each 3d5∕2 and 3d3∕2 component. All cations have Ce3+ valence state. The spectrum (black) of the sample with the (  ×  )R19.1° superstructure is fitted by a weighted linear combination (red) of the CeO2 and Ce2O3 spectra. A cation ratio of 57% Ce3+/43% Ce4+ is determined which corresponds to the Ce7O12 stoichiometry.


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Scitation: Stabilization of the ceria ι-phase (Ce7O12) surface on Si(111)