We have extended the single-impurity cluster model to include the effects of charge transfer to the conduction band. It is found that these effects can have a considerable influence on the line shapes of core-level spectra of small-gap transition-metal compounds. Using a few additional, well-defined parameters to describe the interactions between the local cluster and the conduction band, and retaining the original intracluster parameters from previous single-impurity models, we find an improved agreement for the Ni 2p spectrum of NiS. The asymmetric line shape is well reproduced, keeping the correct satellite to main peak ratio, which is not possible using the three-peak structure of previous models. The new model is less successful in explaining effects beyond standard single-impurity models in more ionic compounds, such as the double-peak stucture of the main line of the core-level spectrum of NiO, where the interactions between the metal d site and the conduction band are expected to play a lesser role.
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