EPR data on the self-interstitial complex $O3\mathrm{}$ in diamond

A previously unreported defect, which is labeled $O3,$ has been observed in the EPR spectrum of synthetic type-IIa diamonds irradiated at 100 K with 2 MeV electrons. This defect was not observed in identical diamonds whose temperature during electron irradiation was ⩾300 K. This center has also been observed in neutron irradiated natural type-Ib diamonds, but only after isochronal annealing at about 650 K: it subsequently annealed out at about 720 K. Analysis of the angular variation of the EPR line positions has determined the zero-field interaction $\left(\underset{¯}{\mathbf{D}}\right),$ showing that the center has a triplet $S=1\mathrm{}$ ground state and $C_2$ symmetry, with a $〈100〉$ rotation axis. The use of a synthetic type-IIa diamond with 5% ${}^{13}\mathrm{C}$ isotopic enrichment allowed observation of ${}^{13}\mathrm{C}$ hyperfine satellites. Analysis of the ${}^{13}\mathrm{C}$ hyperfine couplings by a simple molecular orbital calculation shows that 76% of the unpaired electronic wave function is localized in two nonbonding $2p$ orbitals, on different carbon atoms. Interpretation of the parameter $\underset{¯}{\mathbf{D}}$ as arising primarily from dipole-dipole interaction between these two orbitals indicates that they are separated by 0.32(2) nm. The nonbonding $2p$ orbitals indicate the involvement of $〈100〉$-split interstitials $\left({\mathbf{I}}_{100}\right)$ in the structure. Two possible models are proposed, one involving two and one three parallel ${\mathbf{I}}_{100}$ at next-nearest-neighbor positions, of which the latter gives the better fit to the data.