Charge disproportionation observed by resonant X-ray scattering at the metal–insulator transition in <f>NdNiO3</f>

An epitaxial film of <f>NdNiO3</f> 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 <f>σ–π</f> channel, its weakness and its azimuthal dependence together with a symmetry analysis gives clear indications that the observed energy dependence in the <f>σ–σ</f> channel is not due to the asphericity of the Ni 4p shell, but is directly related to the charge disproportionation. [Copyright &y& Elsevier]