Experimental and theoretical studies of cobalt defects in diamond

Diamonds grown with a cobalt/iron solvent catalyst and annealed at 1800 degrees C were cut and polished along the main symmetry directions. The photoluminescence spectrum showed at least four zero phonon lines. One line at 2.367 eV splits and shifts under uniaxial stress indicating that it originates at a transition between states of A and E symmetry at a trigonal defect. The same samples shelved at least one new electron paramagnetic resonance (EPR) defect, called O4. The EPR spectrum showed a characteristic eight line hyperfine spectrum from a cobalt nucleus with I=7/2. Preliminary measurements suggest the centre has monoclinic symmetry. Ab initio theoretical modelling, using the local density approach (LDA), suggests that the optical centre originates at a cobalt-nitrogen complex with the impurity atoms on neighbouring sites. The structure of the EPR centre suggested from experiment consists of the cobalt atom at the centre of a divacancy, with a neighbouring nitrogen atom. This is supported by the modelling. We have applied three completely independent techniques to cobalt complexes in diamond, and conclude that at least two distinct simple cobalt-nitrogen pairs are present. (C) 2000 Elsevier Science S.A. All rights reserved.