Centres involving two vacancies in diamond

Electron and neutron irradiation of diamond produces a wide variety of different EPR defect centres. The definitive establishment of the vacancy (neutral and negatively charged), the nearest neighbour di-vacancy (R4/W6) and a nearest neighbour pair of [100] split interstitials (Ri) form some starting points, from which to begin to understand what some of the other centres are; by comparison of C-13 hyperfine interactions, and by studying changes in relative concentrations after isochronal annealing. The concentration of R4/W6 rises at temperatures below those at which vacancies are mobile and the intensity of GR1 begins to fall. This suggests that other centres, present at lower temperatures, involve a pair of vacancies, as well as interstitials. and that when they anneal out, they lose the interstitial and end up as R4/W6. Speculations are made about some of these unidentified centres