Electronic changes related to the metal to insulator phase transtion in RNiO3

CÍNTHIA PIAMONTEZE; HÉLIO C. N. TOLENTINO; FLÁVIO C. VICENTIN; NESTOR E. MASSA; JOSE A. ALONSO; MARIA J. MARTINEZLOPE

Surface Review and Letters

World Scientific

Año: 2002 vol. 9 p. 1121 - 1125

0218-625X

Rare earth nickel oxide perovskites (RNiO3, R=rare earth) have, except for LaNiO3, a metal?insulator (MI) phase transition as temperature decreases. The transition temperature (TMI) increases as the Rionbecomes smaller.They present also, at low temperatures, a complex antiferromagnetic order. For lighter Rions(e.g. Pr and Nd),the antiferromagnetic transition temperature (TN) is close to TMI, while for heavier Rions(e.g. Eu, Sm), TMI andTN are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. AlthoughRNiO3 perovskites are placed in the boundary of the Mott?Hubbard and charge transfer regimes, there are severalevidences pointing to a charge transfer gap, mainly controlled by ligand to meta lcharge transfer energy, and thus strongly dependent on hybridization. Ni Ledge absorption spectroscopy (transition 2p → 3d) gives direct information on the density of Ni 3d empty states, and in particular on the multiplet splitting and hybridizationbetween Ni3d and O2p bands. Here we present Ni L3 and L2 absorption spectra measured for NdNiO3 and EuNiO3 (TMI = 200 and 480 K). At room temperature, dramatic differences are observed between EuNiO3 (insulating) andNdNiO3 (metallic). The normalized spectra give evidence for a higher density of 3d unoccupied states and a largermultiplet splitting in EuNiO3. Both effects might be correlated to a decrease in hybridization. The same behavior isobserved for NdNiO3 as it is cooled down to the insulating phase (T < 200 K), revealing that in these compoundsthe opening of the gap is directly related to the degree of hybridization.