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A model is presented for the structure and properties of active centers in lone-pair semiconductors, based on the possibility of unique bonding configurations which can arise from the presence of nonbonding orbitals. It is shown that the lowest-energy neutral center is unstable towards the creation of different positively and negatively charged centers, thus resulting in a negative effective correlation energy. These centers yield gap states which explain the unusual properties of lone-pair semiconductors.
Kastner et al. (Mon,) studied this question.