The infra-red local mode at 1450/cm, H1a, was studied in samples which contained various ratios of 12C and 13C. It had previously been associated with a N interstitial that was assumed to be on an <001> split site. In the present work, the C isotope data showed that a single N atom and 2 equivalent C atoms were involved. Theoretical models showed that the N occupied a position that was mid-way between 2 nearest-neighbor C atoms, and was displaced off the axis between them. The vibration that gave rise to the local mode was that of the N atom perpendicular to the C-C axis. The calculations further showed that the frequency of this mode shifted with isotope substitution, as observed experimentally. There was a metastable structure of the defect in which a C interstitial at an hexagonal site was trapped next to the N substitutional atom. The differences in the stability and migration energies of the negative and neutral charge states of these 2 structures indicated why, in diamond with N aggregates (type-Ia), this defect was formed by irradiation and annealing above 300C. Whereas, in samples with isolated N atoms (type-Ib), irradiation and annealing at above 650C was required.

I.Kiflawi, A.Mainwood, H.Kanda, D.Fisher: Physical Review B, 1996, 54[23], 16719-26