Charge disproportionation inRNiO3perovskites (R=rareearth) from high-resolution x-ray absorption spectroscopy

High-resolution x-ray absorption measurements reveal a rare-earth-dependent splitting of the $\text{Ni}K$ edge in the insulating, charge-disproportionated state of the whole $R{\text{NiO}}_3$ perovskite family. The splitting is five times larger for ${\text{LuNiO}}_3$ [2.5(1) eV] than for ${\text{PrNiO}}_3$ [0.5(3) eV], suggesting that the charge transfer between ${\text{Ni}}^{3+\delta }$ and ${\text{Ni}}^{3-\delta }$ decreases by approaching the itinerant limit and is larger for the heavier lanthanides than suggested in previous studies. The spectroscopic signature of the two Ni sites remains visible above the metal-insulator transition, in agreement with the persistence of dynamic ${\text{Ni}}^{3+\delta }/{\text{Ni}}^{3-\delta }$ charge fluctuations in the metallic phase. This last result generalizes the occurrence of charge disproportionation as alternative to Jahn-Teller distortions to the dynamic regime, giving further support to recent theoretical work [I. I. Mazin, D. I. Khomskii, R. Lengsdorf, J. A. Alonso, W. G. Marshall, R. M. Ibberson, A. Podlesnyak, M. J. Martínez-Lope, and M. M. Abd-Elmeguid, Phys. Rev. Lett. 98, 176406 (2007)].