Nitrogen interstitials in diamond

The infrared local mode at 1450 cm(-1) in diamond, called Hla, has been studied in samples containing mixtures of C-12 and C-13 in several different ratios. It was previously associated with a nitrogen interstitial, 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 that a single nitrogen atom and two equivalent carbon atoms are involved. Theoretical modeling shows that the nitrogen occupies a position midway 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 calculations show that the frequency of this mode shifts with isotope substitution as is observed experimentally. There is a metastable structure of the defect in which a carbon interstitial at a hexagonal site is trapped next to the nitrogen substitutional atom. Differences in the stability and migration energies of the negative and neutral charge states of these two structures indicate why in diamond with aggregated nitrogen (type Ia) this defect is formed by irradiation and annealing above 300 degrees C, while in samples with single nitrogen atoms (type Ib), irradiation and annealing above 650 degrees C is required.