Identification of cobalt on a lattice site in diamond

We report the finding of an electron-paramagnetic-resonance (EPR) center, labeled O4, which contains cobalt on a site in the diamond lattice. O4 is observed at low temperatures $(<\mathrm{}30\mathrm{}\hspace{0.5em}\mathrm{K})$ in high temperature and pressure synthetic diamonds grown with a cobalt containing metal-solvent catalyst after a high-temperature anneal at 1800 °C. The center has $S=1/2,$ monoclinic I symmetry and a hyperfine splitting from a nucleus with $I=7/2.\mathrm{}$ Analysis of the measured hyperfine interaction in terms of ${3d}^5$ gives the unpaired electronic wave function on the cobalt atom, and correctly predicts the measured $\mathbf{g}$ matrix. The wide linewidth of the EPR absorption suggests a small hyperfine interaction arising from a nearby nitrogen atom. Based on this evidence, a model for O4 is presented in terms of a CoN complex.