1ST-PRINCIPLES THEORY OF NITROGEN AGGREGATES IN DIAMOND

Local-density-functional theory is used to evaluate the structure, vibrational bands and electronic properties of two point defects in diamond: a pair of adjacent substitutional N atoms and a vacancy surrounded by four N atoms. These defects are important as they are possible models of the A and B centres seen in type la diamond. For the A centre we find a N-N bond length of 2.14 angstrom, an occupied electronic energy level near midgap and four infrared absorption bands with frequencies close to the experimental values. For the model of the B centre we find a N-C length of 1-49 angstrom; an occupied triplet electronic level within-the gap consistent with the recent suggestion that the N9 absorption band is due to this defect and again vibratory modes in close agreement with experiment. These results provide strong evidence as to the validity of the models for the A and B centres.