Hole distribution between the Ni 3d and O 2p orbitals in Nd<SUB>2−x</SUB>Sr<SUB>x</SUB>NiO<SUB>4−δ</SUB>

We present a joint experimental and theoretical study of x-ray absorption at the O-K and Ni-L2,3 thresholds of Nd2&#8722;xSrxNiO4&#8722;&#948; 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 (XAS) spectra, reflecting holes located in the O 2p orbitals, increases monotonically with the degree of Sr doping up to the maximum doping level studied (x=1.4). 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-L2,3 XAS spectrum of the Ni(III) nickelate Nd1.1Sr0.9NiO3.95 was simulated by a cluster approach, including charge-transfer and complete multiplet interactions, and compared with the corresponding spectrum of the Ni(III) system Nd2Li0.5Ni0.5O4. The 3d7 weight in the ground state of Nd1.1Sr0.9NiO3.95 was found to be 42%, somewhat smaller than the value of 51% found for Nd2Li0.5Ni0.5O4. 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-NiO6-cluster interaction. Such interactions are absent in Nd2Li0.5Ni0.5O4, which is characterized by having isolated NiO6 clusters