Electron paramagnetic resonance investigations of nickel defects in natural diamonds

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectra of natural blue diamonds from the Argyle mine in Western Australia are reported for the first time. These diamonds are shown to contain the NE2 centre which has been observed primarily in synthetic diamonds and has been proposed to be Ni+ with three neighbouring nitrogens. A previously unreported EPR centre is observed and is found to have S = 3/2, g = 2.02 and superhyperfine interaction from one nitrogen. The results are found to be consistent with an Ni- on a substitutional site with an N+ on a fourth nearest neighbour site. This defect is found exclusively in Argyle blue diamonds.