Electron-paramagnetic-resonance measurements on the divacancy defect center R4/W6 in diamond

Electron-paramagnetic-resonance (EPR) studies in radiation damaged diamond enriched to 5% C-13 have resulted in the identification of the nearest-neig hbor divacancy center. It is the isotopic enrichment, and consequent observ ation of C-13 hyperfine lines, that has permitted the structure to be deter mined more than 30 years after the discovery of the center, known as R4 or W6. The center is produced by annealing radiation damaged diamonds to tempe ratures at which the vacancy is mobile (above about 900 K), and in pure dia mond it is the dominant vacancy related product of irradiation and 900 K an nealing. The divacancy anneals out upon prolonged annealing to temperatures above about 1100 K. Low-temperature EPR measurements determine the absolut e sign of the largest principal value of the D matrix, D-3 to be negative; and measurements at temperatures between 4.2 and 300 K indicate that the D matrix is temperature dependent in this interval. The center has C-2h symme try at low temperatures (30 K), and appears to change to axial symmetry abo ut (111) at high temperatures (>400 K). Analysis of the C-13 hyperfine-coup ling data using a simple molecular-orbital model shows that at low temperat ure the unpaired electron probability density is primarily located on four equivalent carbon atoms that are not in the {110} plane of reflection symme try containing the two vacancies. These four carbon atoms show an outward r elaxation around the divacancy. The low-temperature symmetry and localizati on of the unpaired electron probability density is surprising, the former i n the light of theoretical predictions of a (3)A(2g) ground state in the un distorted D-3d symmetry and the latter in comparison with divacancies in si licon. A simple defect molecule calculation suggests that the divacancy has a B-3(u) ground state at low temperatures with C2h symmetry. The large lin ewidth leaves it unclear whether the symmetry changes at high temperatures to D3d The broadening of the EPR linewidth with increasing temperature does not originate from thermally activated reorientation between sites with C- 2h symmetry. It appears to be due to rapid spin-lattice relaxation (via the Orbach mechanism) at temperatures above 50 K, and simple analysis suggests that there is an excited state 20(1) meV above the ground state. [S0163-18 29(99)04620-2].