Electron paramagnetic resonance studies of radiation-damaged samples which had been enriched with 5% 13C resulted in the identification of the nearest-neighbor divacancy center. Isotopic enrichment, and observation of 13C hyperfine lines, permitted the structure to be determined. For instance, the R4 or W6 center was produced by annealing radiation-damaged samples to temperatures at which the vacancy was mobile (above about 900K). In the pure material, it was the predominant vacancy-related product of irradiation and 900K annealing. The divacancy annealed out during prolonged annealing to temperatures above about 1100K. The center had C2h symmetry at 30K, and appeared to change to an axial symmetry about <111> at high temperatures above 400K. Analysis of the 13C hyperfine-coupling data, using a simple molecular-orbital model, showed that the unpaired electron probability density at low temperatures was located mainly on 4 equivalent C atoms that were not in the {110} plane of reflection symmetry containing the 2 vacancies. These four C atoms exhibited an outward relaxation around the divacancy. The low-temperature symmetry and localization of the unpaired electron probability density was considered to be surprising. Firstly because of theoretical predictions of an 3A2g ground state in the undistorted D3d symmetry and secondly because of a comparison with divacancies in Si. A simple defect molecule calculation suggested that the divacancy had a 3Bu ground state at low temperatures, with C2h symmetry. The large line-width made it unclear whether the symmetry changed, at high temperatures, to D3d. The broadening of the electron paramagnetic resonance line-width with increasing temperature did not originate from thermally activated reorientation between sites with C2h symmetry. It appeared to be due to rapid spin-lattice relaxation, via the Orbach mechanism, at temperatures above 50K. A simple analysis suggested that there was an excited state that was 0.02eV above the ground state.

Electron Paramagnetic Resonance Measurements on the Divacancy Defect Center R4/W6 in Diamond. D.J.Twitchen, M.E.Newton, J.M.Baker, T.R.Anthony, W.F.Banholzer: Physical Review B, 1999, 59[20], 12900-10