Interstitial nitrogen and its complexes in diamond

Nitrogen, a common impurity in diamond, can be displaced into an interstitial location by irradiation. The resultant interstitial defects are believed to be responsible for a range of infrared and electronic transitions that vary in thermal stability, and on the type of diamond. Of particular prominence, and the only center for which an atomistic model has been suggested, is the H1a infrared band, which has previously been correlated with the vibration of isolated, bond-centered interstitial N. We present the results of a local-density-functional investigation of interstitial N and a range of complexes made up from N and self-interstitials. We disagree with the previous assignment of H1a to bond-centered interstitial N as we find it is not the ground state structure and it is mobile at temperatures at which H1a is stable. Instead we assign H1a to a complex of two N atoms sharing a single site in a [001]-split configuration, which is both more stable than isolated interstitial N and can simultaneously explain the infrared absorption and dependence on the aggregation stage of N in the irradiated material. We also make tentative assignments for other optical systems thought to involve interstitial N, and suggest schemes for hierarchical formation of complexes for Ia and Ib material.