Hole distribution between the Ni 3d and O 2p orbitals in Nd_2−xSr_xNiO_4−δ

Abstract

We present a joint experimental and theoretical study of x-ray absorption at the O-K and Ni-L_2,3 thresholds of Nd_2−xSr_xNiO_4−δ 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-L_2,3 XAS spectrum of the Ni(III) nickelate Nd_1.1Sr_0.9NiO_3.95 was simulated by a cluster approach, including charge-transfer and complete multiplet interactions, and compared with the corresponding spectrum of the Ni(III) system Nd₂Li_0.5Ni_0.5O₄. The 3d⁷ weight in the ground state of Nd_1.1Sr_0.9NiO_3.95 was found to be 42%, somewhat smaller than the value of 51% found for Nd₂Li_0.5Ni_0.5O₄. 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 Nd₂Li_0.5Ni_0.5O₄, which is characterized by having isolated NiO₆ clusters.