Direct Observation of Charge Order in an Epitaxial ${\mathrm{NdNiO}}_3$ Film

The first direct observation of charge order of ${\mathrm{Ni}}^{3+{\delta }^{\prime }}$ and ${\mathrm{Ni}}^{3-\delta }$ by resonant x-ray scattering experiments in an epitaxial film of ${\mathrm{NdNiO}}_3$ is reported. A quantitative value of $\delta +{\delta }^{\prime }=(0.45\pm 0.04)e$ 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 $R{\mathrm{NiO}}_3$ family.