Hole distribution between the Ni3dand O2porbitals inNd2−xSrxNiO4−δ

Abstract

We present a joint experimental and theoretical study of x-ray absorption at the O-K and $\mathrm{Ni}{\ensuremath{-}L}_{2,3}$ thresholds of ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4\ensuremath{-}\ensuremath{\delta}}$ 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 $\mathrm{Ni}{\ensuremath{-}L}_{2,3}$ XAS spectrum of the Ni(III) nickelate ${\mathrm{Nd}}_{1.1}{\mathrm{Sr}}_{0.9}{\mathrm{NiO}}_{3.95}$ was simulated by a cluster approach, including charge-transfer and complete multiplet interactions, and compared with the corresponding spectrum of the Ni(III) system ${\mathrm{Nd}}_{2}{\mathrm{Li}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_{4}.$ The ${3d}^{7}$ weight in the ground state of ${\mathrm{Nd}}_{1.1}{\mathrm{Sr}}_{0.9}{\mathrm{NiO}}_{3.95}$ was found to be 42%, somewhat smaller than the value of 51% found for ${\mathrm{Nd}}_{2}{\mathrm{Li}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_{4}.$ 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-${\mathrm{NiO}}_{6}$-cluster interaction. Such interactions are absent in ${\mathrm{Nd}}_{2}{\mathrm{Li}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_{4},$ which is characterized by having isolated ${\mathrm{NiO}}_{6}$ clusters.