Diffusion of nitrogen in diamond and the formation of A-centres

• The energetics of defects involved in A-centre formation in diamond are reviewed.
• Mechanisms involving self-interstitials and vacancies are presented.
• A self-interstitial mechanism producing A-centres is shown to be more plausible.

It has long been understood that nitrogen in diamond energetically favours aggregated forms as a consequence of the reduction in the number of dangling bonds. Thus heating to around 1500 °C leads to the formation of A-centres, which are close-by pairs of substitutional N atoms. However, the mechanism by which N moves through the diamond lattice to form the aggregates is much less clear, although for some time it has been generally thought that it may be mediated by lattice vacancies. In this study, we review the energetics of this mechanism in comparison to one based upon self-interstitials, and show that the available experimental evidence is much more consistent with an interstitial mechanism than one based upon lattice vacancies.