A study of C-13 hyperfine structure in the EPR of nickel-nitrogen-containing centres in diamond and correlation with their optical properties

Electron paramagnetic resonance (EPR) and optical spectroscopy have been us ed to determine the structure and electronic state of nickel-nitrogen centr es in natural diamonds and in synthetic diamonds enriched in C-13. The latt er were grown in an Fe-Ni-C solvent/catalyst system at 1750 K, under stabil izing pressure, by the temperature gradient method and afterwards treated a t high temperature and pressure. The parameters and directions of the C-13 hyperfine structure (HFS) tensors for the NEI centre were found to confirm the previously proposed model for this defect, with a nickel ion at the cen tre of a double semivacancy as the basic structural unit. In this unit the nickel atom has six atoms in its coordination shell. The NEI centre has Cah symmetry, and the two equivalent nitrogen atoms in the coordination shell lie in the symmetry plane. New data on the HFS of N-14 and C-13 for the NE5 centre, also with C-2h symmetry, indicated the same structural unit, but t he two equivalent nitrogen atoms (and two equivalent carbon atoms) lie out of the symmetry plane and are related to one another by reflection in it. A new paramagnetic centre was found, labelled NE8, also with C-2h symmetry, with four equivalent nitrogen atoms in the coordination shell all lying out of the symmetry plane. This centre is responsible for the 793.6 nm vibroni c system in absorption and luminescence spectra. The new data have allowed a reinterpretation of the HFS tensors for the NE2 centre, which has C-1 sym metry, suggesting that it has the same structure as NE1 but with one additi onal nitrogen atom in the coordination shell. The electronic states of thes e nickel-containing centres are discussed using the approach of Ludwig and Woodbury to transition metal ions in covalent crystals.