EPR study of the peculiarities of incorporating transition metal ions into the diamond structure

In this paper previously obtained data is reviewed and new data is discussed about nickel-containing centers in diamonds. These data are 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 abundance 100%). A case can be made for a double semi-vacancy structure for these defects. Special growth of diamond in the system enriched in N-15 decreased the line width down to 0.6 G, but gave no direct evidence of the existence of nitrogen in the defect structure. Asymmetrical shapes of the lines 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

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 synthetic diamonds using electron paramagnetic resonance (EPR) and infrared spectroscopy. The diamonds were grown by the temperature gradient HPHT method in a split sphere apparatus using Fe-Ni-C or Fe-Co-C solvent catalysts at 1700 K and 5.5 GPa. We report for the first time the observation of the nitrogen-vacancy (W15) and W33 EPR centres in as-grown and annealed nickel- and cobalt-containing diamonds. The generation of interstitials and vacancies on transformation of substitutional nickel Ni-s(-) into the NE4 defect with the structure of a double semivacancy, and on the reverse transformation, 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 temperature range 1600-1900 K. Nitrogen aggregation observed at higher annealing 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 nearest-neighbour N-N+ and separated nitrogen pairs N---N+ in diamonds. The decreasing content of neutral and positive charge states of nitrogen on X-ray irradiation is due to charge transfer processes between Ni-s(-), P1, A-centres and separated P1 pairs. (C) 2000 Elsevier Science S.A. All rights reserved