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Structural changes in RNiO3 perovskites (R=rare earth) across the metal–insulator transition

2002; Elsevier BV; Volume: 320; Issue: 1-4 Linguagem: Inglês

10.1016/s0921-4526(02)00623-3

1873-2135

Cínthia Piamonteze, H. Tolentino, Aline Y. Ramos, Néstor E. Massa, J. A. Alonso, M. J. Martı́nez-Lope, M. T. Casáis,

Thermal Expansion and Ionic Conductivity

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 R-ion becomes smaller. They also present, at low temperatures, a complex antiferromagnetic order. For lighter R-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature (TN) is close to TMI, while for heavier R-ions (e.g. Eu, Sm), TMI and TN are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. Even though Ni3+ is a Jahn–Teller ion, no distortion in the NiO6 octahedra was found for RNiO3 perovskites with RPr, Nd, Sm and Eu. In this work we have measured EXAFS at Ni K edge for samples of PrNiO3, NdNiO3 and EuNiO3. The Fourier transform spectra for the three samples show a clear splitting in the first peak at the insulating phase. This splitting corresponds to two or more different Ni–O distances. This indicates that there is either a distortion in the NiO6 octahedra or there are two different Ni sites at the insulating phase.