The electronic structure of copper and nickel in the oxidation state qIII was studied for La Li Cu O , Cs KCuF , pronounced preedge peaks that monitor ligand-2p holes induced by covalency. These preedge peaks move to lower energies with increasing oxidation state of Cu and Ni as well as when going from nickelates to cuprates. q 1998 Elsevier Science B.V. All rights reserved.

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Ž .The distribution of valence electrons between transition metals TM and oxygen has been studied via X-ray absorption Ž . near-edge fine structure XANES at the TM-L Ž . concentrations that increase from ; 30% 3d in the Cu III to ; 57% 3d in the Ni III to ; 72% 3d in the Co III compound. Concomitantly, the O-K XANES spectra reflect an increase of bond ionicity in the same sequence. The results show that the Li-doped holes in Nd Li Ni O are distributed between Ni-3d and O-2p orbitals. q 1998 Elsevier Science 2 1r2 1r2 4 B.V. All rights reserved.

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We present a joint experimental and theoretical study of x-ray absorption at the O-K and Ni-L 2,3 thresholds of Nd 2Ϫx Sr x NiO 4Ϫ␦ providing an analysis of the distribution of doped holes induced by Sr substitution between Ni 3d and O 2p orbitals. The preedge peak in the O-K x-ray absorption ͑XAS͒ spectra, reflecting holes located in the O 2p orbitals, increases monotonically with the degree of Sr doping up to the maximum doping level studied (xϭ1.4). The saturation in the relative intensity of the O-K preedge peak in the analogous lanthanum nickelate reported in the literature is shown to be most likely due to surface oxygen deficiency. Furthermore, the experimental Ni-L 2,3 XAS spectrum of the Ni͑III͒ nickelate Nd 1.1 Sr 0.9 NiO 3.95 was simulated by a cluster approach, including charge-transfer and complete multiplet interactions, and compared with the corresponding spectrum of the Ni͑III͒ system Nd 2 Li 0.5 Ni 0.5 O 4 . The 3d 7 weight in the ground state of Nd 1.1 Sr 0.9 NiO 3.95 was found to be 42%, somewhat smaller than the value of 51% found for Nd 2 Li 0.5 Ni 0.5 O 4 . This indicates the influence of nonlocal effects in x-ray-absorption spectroscopy, which, in this case is due to the increased covalency in the Sr-doped system as a result of inter-NiO 6 -cluster interaction. Such interactions are absent in Nd 2 Li 0.5 Ni 0.5 O 4 , which is characterized by having isolated NiO 6 clusters.

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