Electron-paramagnetic-resonance measurements on the di-<001>-split interstitial center (R1) in diamond

Electron-paramagnetic-resonance (EPR) studies in electron-irradiated diamond enriched with 5% C-13 have resulted in identification of the di-[001]-split interstitial center. II is the isotopic enrichment and the consequent observation of C-13 hyperfine satellites that have permitted the structure to be determined more than 30 years after the discovery of the center. Analysis of the C-13 hyperfine couplings by a simple molecular orbital calculation shows that the center may be viewed as a biradical consisting of two unpaired electrons, each primarily localized in a nonbonding 2p orbital, on different carbon atoms. The exchange interaction between these electrons results in a spin-tripler ground state which is well separated from the singlet excited state. The zero-field splitting observed in EPR is dipolar in origin and, taken together with the distribution of the unpaired electron population, indicates that the carbon atoms with the nonbonding 2p orbitals are separated by about 2.2 Angstrom. The center has sigma(h) symmetry. Formation of the di-[001]-split interstitial by room temperature irradiation confirms that the interstitial is mobile at this temperature. Identification of this center suggests a [001]-split interstitial configuration for the carbon self-interstitial, a fundamental defect which has so far eluded direct detection.