Institut für Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg-Universität, D-55099, Mainz, Federal Republic of Germany;
Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Federal Republic of Germany;
Arrhenius Laboratory, Department of Inorganic Chemistry, University of Stockholm, S-10691 Stockholm, Sweden;
Institut für elektronische Eigenschaften, Forschungszentrum, D-52425 Jülich, Federal Republic of Germany

The electronic structures of Eu₂Nb₅O₉, EuNbO₃, and Eu_0.7NbO₃ have been investigated by photoemission and total-yield spectroscopy with synchrotron radiation, and in the case of Eu₂Nb₅O₉ by tight-binding linear muffin-tin orbital (LMTO) band-structure calculations. A central question for reduced europium niobates is that of the valence of Eu and Nb. Both europium and niobium atoms can appear in different valence states so that various electronic configurations in the title compounds are possible. For this reason, the valence band was studied by the resonant Eu 4d(right-arrow)4f technique to determine the Eu valence. The final-state 4f⁶ multiplet of divalent Eu is dominant in all spectra. Since there are no 4f density of states at the Fermi level, valence fluctuations are not expected. The niobium valence states were investigated by core-level spectroscopy. We found only one 3d_5/23d_3/2 doublet for the Nb 3d core level in EuNbO₃ and Eu_0.7NbO₃, while in Eu₂Nb₅O₉, two 3d doublets have been observed, corresponding to two chemically distinct niobium atoms in this compound. The 3d_5/2 peak in EuNbO₃ is assigned to the + 4 nominal valence state at a binding energy of 209.7 eV. The doublet of Eu_0.7NbO₃ is observed at 0.5 eV higher binding energy (at 210.2 eV), which then corresponds to a nominal Nb ^{+ 4 + delta} chemical state. In Eu₂Nb₅O₉, the valence of Nb in the NbO₆ octahedra is less than + 5 and in the Nb₆O₁₂ clusters is close to + 2 as expected. This is in accordance with the LMTO band-structure calculations. © 1998 The American Physical Society.
 

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Physical Review B, 57 1510-1514, 1998.