THEORETICAL INVESTIGATION OF THE ELECTRONIC-STRUCTURE OF NIO BY AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

The electronic structure of NiO near the Fermi level was investigated by ab initio molecular-orbital calculations using the unrestricted Har tree-Fock (UHF) method with basis sets appropriate for nickel and oxyg en atoms in order to explain why NiO behaves as an insulator. In the p resent analysis, we used a cluster composed of finite unit cells of Ni O containing the [Ni-O-6](10-) cluster. We obtained the result that Ni O was an insulator, with a band structure where the upper Ni 3d band w as above the Fermi level, and the upper O 2p, the lower O 2p and the l ower Ni 3d bands were below the Fermi level. These results were in goo d agreement with the experimental results obtained by photoelectron sp ectroscopy (PES) and bremsstrahlung isochromat spectroscopy (BIS), and with the electronic structures derived from the charge-transfer model proposed by Sawatzky and Allen. The main peak observed in PES corresp onds to the d(8) L state, i.e. O 2p band, and the satellite peaks corr espond mainly to the mixed state produced by the correlation between t he 2p state of O atoms and the d(8) state of Ni atoms.