It was noted that N could be displaced into an interstitial location by irradiation, and the resultant interstitial defects were believed to be responsible for a range of infra-red and electronic transitions. In particular, the only center for which an atomistic model was suggested, was the H1a infra-red band, which had previously been correlated with the vibration of isolated bond-centered interstitial N. A local density-functional investigation was made here of interstitial N, and of a range of complexes made up from N and self-interstitials. A previous assignment of H1a to bond-centered interstitial N was questioned, as it was found not to be the ground state structure and was mobile at temperatures at which H1a was stable. It was instead attributed to a complex of two N atoms sharing a single site in a [001]-split configuration. This was both more stable than isolated interstitial N and could simultaneously explain the infra-red absorption, and dependence upon aggregation stage, of N in irradiated material. Tentative attributions were suggested for other optical systems that were thought to involve interstitial N, and schemes were suggested for the hierarchical formation of complexes in Ia and Ib material.

Interstitial Nitrogen and its Complexes in Diamond. J.P.Goss, P.R.Briddon, S.Papagiannidis, R.Jones: Physical Review B, 2004, 70[23], 235208 (15pp)