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Evolution of Electronic Structure Across the Rare-Earth RNiO$_3$ Series

The perovksite rare-earth nickelates, RNiO$_3$ (R=La ... Lu), are a class of materials displaying a rich phase-diagram of metallic and insulating phases associated with charge and magnetic order. Being in the charge transfer regime, Ni$^{3+}$ in octahedral coordination displays a strong hybridization with oxygen to form $3d$ - $2p$ mixed states, which results in a strong admixture of $3d^8\underline{L}$ into $3d^7$, where $\underline{L}$ denotes a hole on the oxygen. To understand the nature of this strongly hybridized ground state, we present a detailed study of the Ni and O electronic structure using high-resolution soft X-ray Absorption Spectroscopy (XAS). Through a comparison of the evolution of the XAS line-shape at Ni L- and O K-edges across the phase diagram, we explore the changes in the electronic signatures in connection with the insulating and metallic phases that support the idea of hybridization playing a fundamental role.