Transition metals in diamond: experimental and theoretical identification of Co-N complexes

Diamonds grown using a cobalt/iron catalyst and annealed at 1800 degrees C were studied using two experimental techniques. A zero-phonon line, observed in photoluminescence at 2.367 eV, showed the splitting under uniaxial stress characteristic of an optical transition at a defect of trigonal symmetry. The same samples were measured by electron paramagnetic resonance (EPR) spectroscopy where they showed the defect labelled O4. This centre contains cobalt and possibly nitrogen, but has monoclinic symmetry. Ab initio modelling using the local density approach (LDA) with the code AIMPRO suggests that the optical transition is at a defect consisting of neighbouring substitutional cobalt and nitrogen atoms. A plausible model for the EPR data is a cobalt atom at the centre of a divacancy with a neighbouring nitrogen atom. We conclude that the defects giving rise to the 2.367 eV photoluminescence and EPR O4 centres are both simple cobalt-nitrogen pairs. (C) 1999 Elsevier Science B.V. All rights reserved.