Correlation between optical absorption and EPR in high-pressure diamond grown from a nickel solvent catalyst

There is a general tendency for the magnitude of the W8 electron paramagnetic resonance, attributed to substitutional negatively charged nickel, Nis−, to increase in sympathy with the strengths of the 1.883 eV and 2.51 eV absorption bands in high-pressure synthetic diamond. The ratio of the intensities of the 1.883–2.51 eV bands is not constant, and is generally lower in diamonds with a low Nis− concentration; a better correlation is obtained between the Nis− concentration and the 2.51 eV absorption than with the 1.883 eV absorption. Concentrations of Nis− up to 70 ppm are detected in some diamonds, and the relatively weak absorption in the 2.51 eV system suggests that either the oscillator strength is very low or that the optical transition occurs at a minor defect with a concentration which increases approximately in direct proportion to the concentration of W8 centres. The 1332 cm−1 absorption induced by nickel, but attributed to N+, does not correlate with Nis− in diamonds with low nitrogen concentrations, in which the concentration of Nis− is also small, and the charge compensation mechanism proposed in earlier investigations needs to be reconsidered for such specimens.