Abstract: The discovery of superconductivity in square planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character and superexchange akin to the cuprates or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role and superexchange is negligible?
Here, we resolve this question using O K-edge and Ni L-edge RIXS measurements of the low valance nickelate La4Ni3O8 and a prototypical cuprate La2−xSrxCuO4 (x=0.35), interpreting the results with exact diagonalization calculations. As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) scheme. The nickelate, however, is not well-described by either limit of the ZSA scheme and is found to be of mixed charge-transfer / Mott-Hubbard character with the Coulomb repulsion U of similar size to the charge transfer energy Δ. Nevertheless, the transition-metal-oxygen hopping is larger in La4Ni3O8 than in La2−xSrxCuO4, which leads to a significant superexchange interaction of J~70 meV and an appreciable hole occupation of the ligand O orbitals in La4Ni3O8 despite its larger Δ. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.