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We report on a Ni L2,3 edges x-ray absorption spectroscopy (XAS) study in RNiO3 perovskites. These compounds exhibit a metal to insulator (M I) transition as temperature decreases. The L3 edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the M I transition in RNiO3 perovskites in terms of modifications in the Ni 3+ crystal field splitting that induces a spin transition from an essentially low-spin (LS) to a mixed-spin state.PACS numbers: 61.10. Ht, 71.30.+h, 75.10.Dg, 75.25.+z Keywords: x-ray absorption spectroscopy, XAS, metal-insulator transition, charge transfer multiplet theory Rare-earth nickel perovskites (RNiO 3 , R=rare earth) present a sharp well-defined metal to insulator (M I) transition as temperature decreases 1 . The transition temperature, T MI , increases with reducing the R ion size, which determines the degree of distortion of the structure 2 . It was proposed that the gap opening would be due to a smaller Ni-O-Ni superexchange angle leading to a reduction of the bandwidth 3 . However, nonnegligible electron-phonon interactions 4 and a shift in T MI with oxygen isotope substitution 5 evidenced the importance of modifications in Ni-O interatomic distances, suggesting a phonon assisted mechanism for conduction. As temperature decreases, these nickelates undergo a magnetic transition to an unusual antiferromagnetic order 6,7,8 . The magnetic arrangement for the lighter R compounds (R=Pr, Nd, Sm, Eu) was refined with a single Ni moment (0.9µ B ) and required non-equivalent couplings among Ni ions to stabilize the structure 6,7 . This is a quite unusual situation in an orthorhombic crystallographic structure whose Ni sites are all equivalent 2 . For the heavier R compounds, Alonso et al.8,9 established a monoclinic distortion in the crystallographic structure leading to two different Ni sites with longer and shorter Ni-O distances alternating along the three axis. The antiferromagnetic structure was explained by a charge ordering defined among the different Ni sites, each one with different magnetic moments (1.4 and 0. low-spin configuration, because the amount of charge transfer for parallel spin almost equals that for antiparallel spin 13 . However, the spectral shape in RNiO 3 compounds is very sensitive to the transition from metallic to insulating states 15 and a complete description of the spin degree of freedom remains to be given. As in recent outcomes on Co 3+ oxides 16,17 , where unconventional spin states exist due to the competition between crystal field splitting and effective 3d exchange interaction, assignments made so far about Ni 3+ in RNiO 3 compounds have to be reexamined.We report here Ni L-edge absorption measurements, which probes directly the available Ni3d states, together with charge transfer multiplet calculations. We establish the importance of the cry...