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 param agnetic resonance, attributed to substitutional negatively charged nic kel, Ni-s(-), to increase in sympathy with the strengths of the 1.883 eV and 2.51 eV absorption bands in high-pressure synthetic diamond. Th e ratio of the intensities of the 1.883-2.51 eV bands is not constant, and is generally lower in diamonds with a low Ni-s(-) concentration; a better correlation is obtained between the Ni-s(-) concentration and the 2.51 eV absorption than with the 1.883 eV absorption. Concentrati ons of Ni-s(-) up to 70 ppm are detected in some diamonds, and the rel atively 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 correl ate with Ni-s(-) in diamonds with low nitrogen concentrations, in whic h the concentration of Ni-s(-) is also small, and the charge compensat ion mechanism proposed in earlier investigations needs to be reconside red for such specimens. (C) 1998 Elsevier Science S.A.