NITROGEN INTERSTITIALS IN DIAMOND

The infrared local mode at 1450 cm(-1) in diamond, called Hla, has bee n studied in samples containing mixtures of C-12 and C-13 in several d ifferent ratios. It was previously associated with a nitrogen intersti tial, assumed to be on a [001] split site [G. S. Woods, Philos. Mag. B 50, 673 (1984)]. In the present work, the carbon isotope data show th at a single nitrogen atom and two equivalent carbon atoms are involved . Theoretical modeling shows that the nitrogen occupies a position mid way between two nearest-neighbor carbon atoms, displaced off the axis between them. The vibration that gives rise to the local mode is that of the nitrogen perpendicular to the carbon-carbon axis, and the calcu lations show that the frequency of this mode shifts with isotope subst itution as is observed experimentally. There is a metastable structure of the defect in which a carbon interstitial at a hexagonal site is t rapped next to the nitrogen substitutional atom. Differences in the st ability and migration energies of the negative and neutral charge stat es of these two structures indicate why in diamond with aggregated nit rogen (type Ia) this defect is formed by irradiation and annealing abo ve 300 degrees C, while in samples with single nitrogen atoms (type Ib ), irradiation and annealing above 650 degrees C is required.