Character of the insulating state in NiO: A mixture of charge-transfer and Mott-Hubbard character

Using site-specific soft x-ray emission and absorption spectroscopy in conjunction with site-specific x-ray photoelectron spectroscopy, we present measurements of the valence and conduction band states of NiO. Significant hybridization between the Ni and the O electronic states near the Fermi energy is observed throughout both the valence and conduction bands, in accordance with density-functional theory calculations also presented. The relationship between our emission and absorption measurements also shows direct similarities to other techniques used to measure the non-dipole-allowed d-d transitions within NiO, addressing questions concerning the character of the insulating band gap of this system. NiO has previously been described as a large-gap, charge-transfer insulator, while our data show the valence and conduction states at the Fermi energy to be primarily of Ni d-character hybridized with a small amount of O p-character states, suggesting a mixture of charge-transfer and Mott-Hubbard character as described by the Zaanen-Sawatzky-Allen model.