Crystal-field excitations in NiO studied with hard x-ray resonant inelastic x-ray scattering at the NiK edge

Charge-neutral dd excitations in NiO have been studied using resonant inelastic x-ray scattering (RIXS) at the quadrupolar pre-edge of the Ni K absorption edge (8331 eV) with an energy resolution of 250 meV. The results agree well with calculations performed using the atomic multiplet model for the Ni2+ ion including crystal-field effects. Most notably the first t(2g)-> e(g) excitation at 1.05 eV was observed clearly and its energy determined with a high accuracy. The remaining differences between the theoretical and the experimental results, especially in the incident-photon-energy dependence, are suggested to be due to lattice distortions. This study demonstrates that bulk sensitive, high-resolution hard-x-ray RIXS can open up possibilities in studies of highly correlated materials.