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 thes2pchannel, its weakness and its azimuthal dependence together with a symmetry analysis gives clear indications that the observed energy dependence in thes2schannel is not due to the asphericity of the Ni 4p shell, but is directly related to the charge disproportionation.