Electron paramagnetic resonance and optical spectroscopy were used to study Ni-N centres in natural diamonds, and in synthetic diamonds which were enriched in 13C. The latter were grown under pressure in an Fe-Ni-C solvent/catalyst system at 1750K, using the temperature-gradient method. The parameters and directions of the 13C hyperfine structure tensors for the NE1 centre were found to confirm a previously proposed model for this defect; which comprised a Ni ion at the centre of a double semi-vacancy. In this unit, the Ni atom had 6 atoms in its coordination shell. The NE1 centre had C2h symmetry, and the 2 equivalent N atoms in the coordination shell lay in the symmetry plane. New data on the hyperfine structure of 14N and 13C for the NE5 centre, which also had C2h symmetry, suggested the same structural unit, but the 2 equivalent N atoms (and 2 equivalent C atoms) lay outside of the symmetry plane and were related to each other by reflection. A new paramagnetic centre, NE8, was found which also had C2h symmetry; with 4 equivalent N atoms in the coordination shell - all of which lay outside of the symmetry plane. This centre was responsible for the 793.6nm vibronic system in the absorption and luminescence spectra. The new data permitted a reinterpretation of the hyperfine structure tensors for the NE2 centre, which had C1 symmetry; thus suggesting that it possessed the same structure as NE1 but had an additional N atom in the coordination shell.

A Study of 13C Hyperfine Structure in the EPR of Nickel-Nitrogen-Containing Centres in Diamond and Correlation with their Optical Properties. V.A.Nadolinny, A.P.Yelisseyev, J.M.Baker, M.E.Newton, D.J.Twitchen, S.C.Lawson, O.P.Yuryeva, B.N.Feigelson: Journal of Physics - Condensed Matter, 1999, 11[38], 7357-76