Direct observation of charge order in an epitaxial NdNiO3 film

The first direct observation of charge order of Ni3+delta' and Ni3-delta by resonant x-ray scattering experiments in an epitaxial film of NdNiO3 is reported. A quantitative value of delta + delta' = (0.45 +/- 0.04)e was obtained. The temperature dependence of the charge order deviates significantly from those of the magnetic moment and crystallographic structure. This might be an indication of a difference in their fluctuation time scales. These observations are discussed in terms of the temperature-driven metal-insulator transition in the RNiO3 family.