The growth rate effect on the nitrogen aggregation in HTHP grown synthetic diamonds

Plates cut out parallel to the (110) plane from high quality synthetic diamond crystals grown in the Fe-Ni-C system by a temperature gradient method at pressure 6 Gpa and temperature 1500-1550 degrees C were comprehensively studied. Analysis of the sector structure allows determination of the average and relative growth rates of each octahedral growth sector. Concentrations of nitrogen-related C, A, and Ni centers in some profiles of different octahedral growth sectors were analyzed by FTIR spectroscopy. The regularities of the spatial distribution of A-defects are consistent with the assumption that A-defects were formed in the crystals as a result of annealing during crystal growth. It was found that A-defect formation proceeds more intensively in octahedral growth sectors which were formed at higher linear growth rates. Possible mechanisms for the growth rate effect on the nitrogen aggregation process in synthetic diamonds are proposed. (C) 2000 Elsevier Science S.A. All rights reserved

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

A novel use of hyperfine structure in the electron paramagnetic resonance of interacting pairs of paramagnetic defects in diamond

It is the hyperfine structure of N-14 and C-13 in the electron paramagnetic resonance (EPR) spectrum which indicates that the unpaired electron of a single substitutional nitrogen atom in diamond is in one of the four anti-bonding N-C orbitals. We show that, for diamonds containing a very high concentration of nitrogen, the hyperfine structure of interacting pairs of nitrogen atoms indicates that for close neighbours there are unique orientations of the constituent N-C bonds, while at larger distances the orientations are random

New EPR spectra in diamonds with a high concentration of nitrogen atoms

The EPR spectrum of a synthetic diamond, containing a high concentration of isolated substitutional nitrogen, N-S (the P1 EPR centre), shows in addition to the spectrum of P1 three other features not previously observed in nitrogen containing diamond:1. an 'allowed' pair spectrum close to that of P1 corresponding to interacting pairs of N-S at three sites separated by 0.357-0.564 nm;2. a 'forbidden' pair spectrum at about half of the magnetic field required for P1, with hyperfine structure characteristic of a pair of interacting nuclei with I = 1, each with the same hyperfine parameters as P1: corresponding to a superposition of many pairs of separation greater than 0.7 nm;3. a double quantum spectrum of P1, being an exact replica of the normal spectrum at twice the magnetic field. (C) 1999 Elsevier Science S.A. All rights reserved