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

Abstract Resonant X-ray scattering data on an epitaxial film of NdNiO 3 are presented. Below the metal to insulator transition a clear energy dependence is observed at the Ni K-edge, whereas above this temperature this energy dependence is absent. These data give clear evidence of a charge disproportionation on the Ni site, leading to two different electronic Ni ions. No indication of orbital ordering has been found so far.