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  • 1.
    Castleton, Christopher
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Kullgren, J.
    Department of Materials Chemistry, Uppsala University, Box 538, SE-75121, Sweden.
    Hermansson, K.
    Department of Materials Chemistry, Uppsala University, Box 538, SE-75121, Sweden.
    Tuning LDA+U for electron localization and structure at oxygen vacancies in ceria2007In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, no 24Article in journal (Refereed)
    Abstract [en]

    We examine the real space structure and the electronic structure (particularly Ce4f electron localization) of oxygen vacancies in Ce O2 (ceria) as a function of U in density functional theory studies with the rotationally invariant forms of the LDA+U and GGA+U functionals. The four nearest neighbor Ce ions always relax outwards, with those not carrying localized Ce4f charge moving furthest. Several quantification schemes show that the charge starts to become localized at U˜3 eV and that the degree of localization reaches a maximum at ~6 eV for LDA+U or at ~5.5 eV for GGA+U. For higher U it decreases rapidly as charge is transferred onto second neighbor O ions and beyond. The localization is never into atomic corelike states, at maximum localization about 80-90% of the Ce4f charge is located on the two nearest neighboring Ce ions. However, if we look at the total atomic charge we find that the two ions only make a net gain of (0.2-0.4) e each, so localization is actually very incomplete, with localization of Ce4f electrons coming at the expense of moving other electrons off the Ce ions. We have also revisited some properties of defect-free ceria and find that with LDA+U the crystal structure is actually best described with U=3-4 eV, while the experimental band structure is obtained with U=7-8 eV. (For GGA+U the lattice parameters worsen for U>0 eV, but the band structure is similar to LDA+U.) The best overall choice is U˜6 eV with LDA+U and ˜5.5 eV for GGA+U, since the localization is most important, but a consistent choice for both Ce O2 and Ce2 O3, with and without vacancies, is hard to find. © 2007 American Institute of Physics.

  • 2.
    Castleton, Christopher
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Nokbin, S.
    Materials Chemistry, The Ångström Laboratory, Box 538, SE-751 21 Uppsala, Sweden, Laboratory for Computational and Applied Chemistry, Department of Chemistry, Kasetsart University, Bangkok, 10900, Thailand.
    Hermansson, K.
    Materials Chemistry, The Ångström Laboratory, Box 538, SE-751 21 Uppsala, Sweden.
    Correlations between magnetic properties and bond formation in Rh-MgO(0 0 1)2007In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, no 5, p. 1218-1230Article in journal (Refereed)
    Abstract [en]

    We present the results of first principles calculations for the magnetism of Rh adlayers on MgO(0 0 1), at three different adsorption sites and three different coverages, corresponding to 1, 1/2 and 1/8 monolayers. Finite magnetization is found in all cases except that of one Rh monolayer above the oxygen site, which is also the most stable. We examine how the magnetization changes as a function of the Rh-surface distance and relate this to changes in the real-space charge density and in the density of states (DOS) as the Rh adlayer interacts with the surface. We find that increasing either the Rh-Rh interaction strength or the Rh-surface interaction strength leads to reduced magnetization, while increasing the former drives a crossover from localized (atomic) to itinerant magnetism. Neither the magnetic transition itself, nor the localized-to-itinerant magnetism crossover, is found to be directly related to the formation of Rh-surface bonds. From a practical point of view, we predict that magnetism in the Rh-MgO(0 0 1) system is most likely to be found experimentally at reduced coverages and at low temperatures. © 2006 Elsevier B.V. All rights reserved.

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