Abstract
Numerous samples of the system La2−xSrxCu1−yRuyO4−δ (0.86≤x≤2, 0.05≤y≤1) as well as several other cuprates and ruthenates have been investigated by XANES spectroscopy. The exact energy positions of the white lines observed at the Cu– and Ru– LIII absorption edges are a sensitive measure for the oxidation state of the corresponding transition metal. Our results lead to the following fundamental conclusions: (i) Charge compensation in the system La2−xSrxCu1−yRuyO4−δ is achieved by a successive oxidation of Ru4+ to Ru5+, while copper remains in the oxidation state 2+. (ii) The formation of Ru6+ ions or higher oxidized species can be ruled out. (iii) For those compounds in which charge neutrality is not possible by the formation of Ru5+ alone, the remaining electron “holes” are located at the oxygen sites. The possibility of using the valence shift as a quantitative measure for the oxidation state of Ru in new compounds is briefly discussed.
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