Charge disproportionation and search for orbital ordering in NdNiO3 by use of resonant x-ray diffraction

Abstract

Detailed resonant x-ray diffraction experiments including azimuthal angle scans were performed on NdNiO3 around the Ni K absorption edge, allowing us to investigate the electronic changes associated with the metal-to-insulator transition. The influence on the observed reflections of charge disproportionation and the asphericity of the Ni electron density is evaluated. The asphericity, due to the distortion of the oxygen octahedra, persists also in the metallic phase, but its influence on the unrotated σ-σ (033) diffraction signal is found to be negligible, in contrast to the case for the superlattice reflections found in manganites and magnetite. We conclude that the σ-σ scattering intensity is consistent with charge disproportionation at the Ni site. No (1 201 2) type reflections were found, making unlikely the simultaneous occurrence of orbital order with this previously proposed wave vector. Moreover, the asphericity observed in the 4p shell in the metallic state indicates that there is no orbital degeneracy. Therefore, no orbital ordering is expected to occur at the metal-insulator transition. © 2005 The American Physical Society.