Synthetic crystals were grown using a solvent catalyst that contained Ni, and were studied by means of the optical detection of electron paramagnetic resonance; using the magnetic circular dichroism  of the optical absorption. The latter spectra, in the infra-red spectral region, consisted of a derivative-like line at 1.06eV, single sharp lines at 1.29 and 1.40eV, a complex phonon-split band with the zero phonon line at 1.69eV and a derivative-like doublet structure which was superposed at 1.72eV. All of these optical absorptions were shown to originate from paramagnetic defects. Using optically detected electron paramagnetic resonance, it was possible to unambiguously attribute the 1.72eV doublet to the NE4 centre, and the 1.40eV line to the NIRIM-2 centre. The defect which was responsible for the zero phonon line at 1.06eV had the same symmetry as the NIRIM-2 centre, with a slightly larger g||-value. The NE4 centre was attributed to substitutional Ni+ which was associated with a C vacancy in a nearest-neighbour position, while the NIRIM-2 centre was attributed to interstitial Ni+ with a distortion along a [111] axis.

Optically Detected Electron Paramagnetic Resonance of Ni-Related Defects in Synthetic Diamond Crystals. T.Pawlik, C.Noble, J.M.Spaeth: Journal of Physics - Condensed Matter, 1998, 10[43], 9833-40