Crystal-field excitations in NiO under high pressure studied by resonant inelastic x-ray scattering.

We report a study on charge-neutral crystal-field (dd) excitations in NiO as a function of applied pressure up to 55 GPa, using resonant inelastic x-ray scattering spectroscopy at the Ni K edge. We find distinct signatures of the pressure-induced modifications to the 3d orbital energies as a function of pressure. These modifications are experimentally evidenced by a subtle splitting of the dd-excitation resonance energies.