Abstract
We report ultraviolet and x-ray-photoelectron spectroscopy and electron-energy-loss spectroscopy results for clean LaCoO/sub 3/ and LaCoO/sub 3/ surfaces covered with chemisorbed oxygen at room temperature. Also the temperature dependence of the HeII spectrum (h..nu..=40.8 eV) for clean LaCoO/sub 3/ was monitored between approx.140 and 1050 K. We found that chemisorbed oxygen caused significant changes in the surface electronic structure of LaCoO/sub 3/ near the Fermi energy, as did changing temperature. Temperature-dependent electronic properties are expected, based on the known bulk low-spin--high-spin conversion of the trivalent d/sup 6/ Co ions, as has been successfully addressed by Goodenough. The temperature dependences we observed, while consistent with Goodenough's bulk model, could also be understood if the ground state of the surface Co ions were reversed from that of the bulk. We suggest that such a reversal could be driven by a small positional relaxation (expansion) of the top atomic layers of the material. Finally we discuss the implications of the chemisorption results and the postulated surface relaxation in light of the Voorhoeve picture of the high catalytic activity of LaCoO/sub 3/ for CO oxidation.
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