Mechanisms of nitrogen aggregation in nickel- and cobalt-containing synthetic diamonds

We present a study of the point defects observed in as-grown and annealed s ynthetic diamonds using electron paramagnetic resonance (EPR) and infrared spectroscopy. The diamonds were grown by the temperature gradient HPHT meth od in a split sphere apparatus using Fe-Ni-C or Fe-Co-C solvent catalysts a t 1700 K and 5.5 GPa. We report for the first time the observation of the n itrogen-vacancy (W15) and W33 EPR centres in as-grown and annealed nickel- and cobalt-containing diamonds. The generation of interstitials and vacanci es on transformation of substitutional nickel Ni-s(-) into the NE4 defect w ith the structure of a double semivacancy, and on the reverse transformatio n, respectively, and the existence of different charge states of nickel and nitrogen defects, are reasons for aggregation of nitrogen at low annealing temperature. Most of the Ni; is transformed into Ni-N complexes in the tem perature range 1600-1900 K. Nitrogen aggregation observed at higher anneali ng temperatures is due to a third mechanism by enhancement of the mobility of Ni by the Coulomb field of negatively charged nickel-containing centres. Charge transfer induced by X-ray irradiation indicated the existence of ne arest-neighbour N-N+ and separated nitrogen pairs N---N+ in diamonds. The d ecreasing content of neutral and positive charge states of nitrogen on X-ra y irradiation is due to charge transfer processes between Ni-s(-), P1, A-ce ntres and separated P1 pairs. (C) 2000 Elsevier Science S.A. All rights res erved.