Charge disproportionation observed by resonant X-ray scattering at the metal-insulator transition in NdNiO3

An epitaxial film of NdNiO3 was investigated by the resonant X-ray scattering technique. Below the metal-to-insulator transition a pronounced energy dependence of the scattering intensity at the Ni K-edge is observed. This is clear evidence for a charge disproportionation on the Ni site, leading to two different electronic Ni ions. The occurrence of a reflection in the sigma-pi channel, its weakness and its azimuthal dependence together with a symmetry analysis gives clear indications that the observed energy dependence in the sigma-sigma is not due to the asphericity of the Ni 4p shell, but is directly related to the charge disproportionation.