The effect of nickel and cobalt on the aggregation of nitrogen in diamond

The effect of nickel and cobalt impurities on nitrogen aggregation in high-pressure, high-temperature (HPHT)-grown synthetic diamonds has been investigated. Samples have been grown from solvent-catalysts of different nickel-iron ratios to vary the nickel content. Experimentally, a marking method has been developed to allow accurate repositioning of samples for infra-red absorption measurements at each stage of the annealing, and samples have been carefully prepared to exclude the possibility of growth-sector overlap and to minimise effects due to inhomogeneity within a growth-sector. The results indicate an enhancement in {111} growth sectors due to the preferential incorporation of nickel, with the degree of enhancement increasing with increasing nickel content. A marked deviation from simple second order kinetics was found for the enhanced cases. Similar results were found for {111} sectors of cobalt/iron-grown samples. Features in infra-red and visible absorption spectra allow a model for the aggregation enhancement via interstitial atoms to be proposed. (C) 1998 Elsevier Science S.A.