Charge transfer at very high pressure in NiO

We use resonant inelastic x-ray scattering to study the electronic structure of nickel oxide, the prototype charge-transfer insulator, as a function of pressure. At ambient pressure, we observe spectral features due to the charge-transfer excitation and the Coulomb correlation energy which progressively smear up to a pressure of 100 GPa. These changes are interpreted as due to increased dispersion of the concerned electronic bands. This opens new perspectives both for the study of electronic structure of correlated materials and for high-pressure research such as a deeper understanding of metal-insulator transitions, as well as the study of deep-earth chemistry.