Band gap in NiO: A cluster study

Abstract

The Mott-Hubbard gap U and the charge-transfer gap Δ of solid NiO are estimated from ab initio calculations on the ${\mathrm{NiO}}_6^{10\mathrm{-}}$ cluster. Covalency in the essentially localized $d^n$ states and localization for the spatially extended O(2p) hole states are introduced by means of a limited configuration-interaction calculation. The localized states induce a large polarization effect in the bulk, accounted for in a semiempirical way. The values obtained for U and Δ are quite similar and in the range of 4.4–5.2 eV, in good agreement with the observed gap.