ELECTRONIC CHANGES RELATED TO THE METAL-TO-INSULATOR PHASE TRANSITION IN RNiO3

Abstract

Rare earth nickel oxide perovskites (R NiO 3, R=rare earth) have, except for LaNiO 3, a metal–insulator (MI) phase transition as temperature decreases. The transition temperature (T MI ) increases as the R-ion becomes smaller. They present also, at low temperatures, a complex antiferromagnetic order. For lighter R-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature (T N ) is close to T MI , while for heavier R-ions (e.g. Eu, Sm), T MI and T N are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. Although R NiO 3 perovskites are placed in the boundary of the Mott–Hubbard and charge transfer regimes, there are several evidences pointing to a charge transfer gap, mainly controlled by ligand-to-metal charge transfer energy, and thus strongly dependent on hybridization. Ni L-edge absorption spectroscopy (transition 2p → 3d) gives direct information on the density of Ni 3d empty states, and in particular on the multiplet splitting and hybridization between Ni 3d and O 2p bands. Here we present Ni L3 and L2 absorption spectra measured for NdNiO 3 and EuNiO 3 (T MI = 200 and 480 K). At room temperature, dramatic differences are observed between EuNiO 3 (insulating) and NdNiO 3 (metallic). The normalized spectra give evidence for a higher density of 3d unoccupied states and a larger multiplet splitting in EuNiO 3. Both effects might be correlated to a decrease in hybridization. The same behavior is observed for NdNiO 3 as it is cooled down to the insulating phase (T < 200 K), revealing that in these compounds the opening of the gap is directly related to the degree of hybridization.