Evolution of electronic structure across the rare-earth RNiO₃ series

The perovksite rare-earth nickelates, RNiO₃ (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, Ni³⁺ in octahedral coordination displays a strong hybridization with oxygen to form 3d-2p mixed states, which results in a strong admixture of 3d⁸L into 3d⁷, where 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.