THE OPTICAL GAP OF NIO

The nature of the optical absorption gap in NiO at 4.0 eV is investigated. It is found that this gap is due to a band to band transition, where an electron is taken out of the valence band and placed into the conduction band. The optical gap of 6.0 eV found in Ni:MgO is of a nature, where an electron is taken out of the oxygen 2p band and placed into the first affinity level of the Ni2+ ion (3 d8 L --> 3 d9 L-1). The impurity band created in Ni1-xLix O by the Li ions is found 2.3 eV below the bottom of the conduction band in agreement with model predictions.