EPR data on the self-interstitial complex O3 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 greater than or equal to 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 (D), showing that the center has a triplet S=1 ground state and C-2 symmetry, with a (100) rotation axis. The use of a synthetic type-IIa diamond with 5% C-13 isotopic enrichment allowed observation of C-13 hyperfine satellites. Analysis of the C-13 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 (D) under bar 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 (I-100) in the structure. Two possible models are proposed, one involving two and one three parallel I-100 at next-nearest-neighbor positions, of which the latter gives the better fit to the data.