By using a thin NiO single-crystal sample, it has been possible to observe electroreflectance spectra between 3.0 and 5.0 eV. The results of this study and a recent photoconductivity investigation suggest that an energy-band description which includes the $3d$ electrons is necessary for a full description of NiO interband optical properties.

• Received 31 July 1969

DOI:https://doi.org/10.1103/PhysRevLett.23.915