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Tuning LDA+U for electron localization and structure at oxygen vacancies in ceria
KTH, School of Biotechnology (BIO), Theoretical Chemistry.
2007 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, no 24Article in journal (Refereed) Published
Abstract [en]

We examine the real space structure and the electronic structure (particularly Ce4f electron localization) of oxygen vacancies in CeO2 (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 approximate to 3 eV and that the degree of localization reaches a maximum at similar to 6 eV for LDA+U or at similar to 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 approximate to 6 eV with LDA+U and approximate to 5.5 eV for GGA+U, since the localization is most important, but a consistent choice for both CeO2 and Ce2O3, with and without vacancies, is hard to find.

Place, publisher, year, edition, pages
2007. Vol. 127, no 24
Keyword [en]
generalized gradient approximation, strongly correlated systems, density-functional theory, augmented-wave method, low-index surfaces, small-polaron, transport-properties, defect structure, reduced ceria, hubbard-model
URN: urn:nbn:se:kth:diva-17190DOI: 10.1063/1.2800015ISI: 000251987800030OAI: diva2:335233
QC 20100525Available from: 2010-08-05 Created: 2010-08-05Bibliographically approved

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