Metal-Oxygen Hybridization and Core-Level Spectra in Actinide and Rare-Earth Oxides
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Metal-Oxygen Hybridization and Core-Level Spectra in Actinide and Rare-Earth Oxides Jindřich Kolorenč Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 18221 Praha, Czech Republic ABSTRACT We employ a combination of the density-functional theory and the dynamical mean-field theory to study the electronic structure of selected rare-earth sesquioxides and dioxides. We concentrate on the core-level photoemission spectra, in particular, we illustrate how these spectra reflect the integer or fractional filling of the 4f orbitals. We compare the results to our earlier calculations of actinide dioxides and analyze why the core-level spectra of actinide compounds display a substantially reduced sensitivity to the filling of the 5f orbitals. INTRODUCTION Recently, we have used a combination of the density-functional theory and the dynamical mean-field theory, the so-called LDA+DMFT method, to investigate the electronic structure of three actinide dioxides, UO2, NpO2, and PuO2 [1]. The theory indicates a large covalent mixing of the actinide 5f states with the 2p states of oxygen, which induces a substantially increased filling of the 5f orbitals away from the nominal integer occupation. The core-level spectroscopy is able to detect a non-integer number of 4f electrons n4f in many rare-earth compounds, including dioxides CeO2 and PrO2, and to distinguish this fractional filling from an integer filling that is characteristic, for instance, to rare-earth sesquioxides [2,3]. The core-level spectra of the actinide dioxides, on the other hand, appear to be compatible with an integer number of 5f electrons [4]. For example, the 4f x-ray photoemission spectra (XPS) show only small shakeup satellites, whereas the 3d XPS in CeO2, in which there is approximately one half of an electron in the 4f shell, has a very different three-peak shape. We argue that this difference does not point to an integer number of 5f electrons in the actinide dioxides but to a reduced sensitivity of the core-level spectra to the filling of the 5f orbitals. Our large deviations from the nominal integer filling are in fact compatible with the small shake-up satellites in 4f XPS when a detailed calculation is performed [1]. To put our theoretical findings on an even firmer ground, we apply the LDA+DMFT method to the electronic structure of selected rare-earth oxides with known fractional and near-integer n4f , and analyze possible reasons for the reduced sensitivity of the core spectra to the 5f electron count in the actinide oxides. COMPUTATIONAL METHOD We employ the implementation of the LDA+DMFT method described in [1]. First, the nonmagnetic LDA band structure, obtained with the aid of the WIEN2K code [5], is represented by a tight-binding hamiltonian in the basis of Wannier functions with lanthanide 6s, 4f, 5d, and oxygen 2p character [6,7]. Then, a spherically symmetric Coulomb vertex, parametrized by four Slater integrals ( F0 , F2 , F4 , and F6 ), is added to each of the 4f shells, and the resulting Hubbard model is solved using the dynami
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