Electron paramagnetic resonance (EPR) and optical spectroscopy have been used to determine the structure and electronic state of nickel-nitrogen centres in natural diamonds and in synthetic diamonds enriched in 13C. The latter were grown in an Fe-Ni-C solvent/catalyst system at 1750 K, under stabilizing pressure, by the temperature gradient method and afterwards treated at high temperature and pressure. The parameters and directions of the 13C hyperfine structure (HFS) tensors for the NE1 centre were found to confirm the previously proposed model for this defect, with a nickel ion at the centre of a double semivacancy as the basic structural unit. In this unit the nickel atom has six atoms in its coordination shell. The NE1 centre has C2h symmetry, and the two equivalent nitrogen atoms in the coordination shell lie in the symmetry plane. New data on the HFS of 14N and 13C for the NE5 centre, also with C2h symmetry, indicated the same structural unit, but the 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 C2h 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 vibronic system in absorption and luminescence spectra. The new data have allowed a reinterpretation of the HFS tensors for the NE2 centre, which has C1 symmetry, suggesting that it has the same structure as NE1 but with one additional nitrogen atom in the coordination shell. The electronic states of these nickel-containing centres are discussed using the approach of Ludwig and Woodbury to transition metal ions in covalent crystals.

Abstraet.In this paper previously obtained data is reviewed and new data is discussed about nickelcontaining centers in diamonds. These data ate used to suggest interpretation of new data about cobalt-containing centers and to understand the influence of iron on the defects in diamonds grown in the iron system. A newly discovered nickel-nitrogen center has three nitrogen atoms in the first neighbor sphere around the double semi-vacancy and looks like the N3 (P2) center. In diamonds grown in the cobalt system two new types of cobalt-containing centers were found (NLO2 and NWO1). Both centers have electron spin S = 1/2 and hyperfine structure from one cobalt ion (I = 7/2 with natural abun:ldnce 100%). A case can be made fora double semi-vacancy structure for these defects. Special"~i-owth of diamond in the system enriched in tSN decreased the line width down to 0.6 G, but gave no direct evidence of the existence of nitrogen in the defeet structure. Asymmetrical shapes of the hnes in the electron paramagnetic resonance (EPR) spectra of cobalt-containing centers with opposite signs in low and high magnetic field parts of spectra are due to very sensitive spin-Hamiltonian parameters of these defects to the lattice distortions. Annealing of cobalt-containing crystals at 2600 K produces the disappearance of all cobalt-containing EPR spectra, probably due to the capture of an additional nitrogen atom and the creation of a 3d 6 diamagnetic state. In diamonds grown in the iron system with a high content of nitrogen there is evidence of an influence of ferromagnetic inclusion on the exchange interaction between substitutional nitrogen as an additional channel of indirect exchange interaction. lntroductionThe development of the technology of both synthesis and annealing of diamond crystals allows the study of a means of successively incorporating impurity ions into the diamond structure. Progress in understanding the formation of nitrogen defects created in diamonds began with the seminal papers of Chrenko et al. [1]

Electron paramagnetic resonance (EPR) was used to investigate the transformation of as grown nickel and nitrogen defects at the armealing of synthetic diamonds, obtained by the temperature gradient method. Structural models and formation mechanisms of the seven nickel containing paramagnetic centres (NE1-NE7) in synthetic diamonds ate discussed. A common structural fragment of NE1-NE4 centres is a double semivacaney, in centre of which Ni + ion is located. The NE6, NE7 centres are proved to operate as shallow electronic traps. The effects observable for NE5 centres ate supposed due to ah intemal electronic transformation in them. The problem of eharge compensators for nickel ions is also discussed. The features observable in the charge transfer processes under X-ray irradiation for niekel and nitrogen containing centres suggest that donor nitrogen serves asa bulk charge eompensator for substitutional nickel and NE1 centres.

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