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.

• Received 13 May 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.1504