MODELS FOR THE DI-NITROGEN CENTERS FOUND IN BROWN DIAMOND

No new experiments are reported in this paper: the paper comprises a re-interpretation of published EPR data on the various di-nitrogen centres in brown diamond, including the new ENDOR results of our previous paper on the W7 centre. The new evidence about W7 provided by these measurements, particularly the identification of the site symmetry as C1, together with other data about the centre, has enabled us to propose a definitive model for W7: a puckered ring structure (comprising four C atoms, one N atom and N+ ion). This model for the W7 centre is shown to lead, by comparison of the properties of the two centres, to a second neighbour, N-C-N+, model for the N1 centre. This model for N1 is shown, in the appendix, to allow new information to be drawn from C-13 hyperfine structure in the EPR spectrum of P1, the single substitutional nitrogen. A re-examination of published experimental EPR data of Welbourn for the N4 centre, showing that the two nitrogen atoms are equivalent, is used to propose a collinear N-C-+C-N model for N4. The occurrence of only some of the possible orientations of W7 and N4 centres allowed by the symmetry operations of the N4 centre, are explained in terms of a model of the formation of these two centres by plastic deformation.