ELECTRONIC CHANGES RELATED TO THE METAL-TO-INSULATOR PHASE TRANSITION INRNiO3

Rare earth nickel oxide perovskites (RNiO3,R=rare earth) have, except forLaNiO3, a metal–insulator (MI) phase transition as temperature decreases. The transition temperature(TMI)increases as theR-ion becomes smaller. They present also, at low temperatures, a complex antiferromagnetic order. For lighterR-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature(TN)is close toTMI, while for heavierR-ions (e.g. Eu, Sm),TMIandTNare very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. AlthoughRNiO3perovskites 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. NiL-edge absorption spectroscopy (transition2p → 3d) gives direct information on the density of Ni3dempty states, and in particular on the multiplet splitting and hybridization betweenNi3dandO2pbands. Here we present NiL3andL2absorption spectra measured forNdNiO3andEuNiO3(TMI= 200and 480 K). At room temperature, dramatic differences are observed betweenEuNiO3(insulating) andNdNiO3(metallic). The normalized spectra give evidence for a higher density of3dunoccupied states and a larger multiplet splitting inEuNiO3. Both effects might be correlated to a decrease in hybridization. The same behavior is observed forNdNiO3as it is cooled down to the insulating phase(T