Direct Observation of Charge Order in an Epitaxial NdNiO3 Film

The first direct observation of charge order of Ni**(3+delta ') and Ni**(3-delta) by resonant X-ray scattering experiments in NdNiO_3 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 RNiO_3 family.