A singly ionized N-C-N centre in diamond

Abstract

The ESR spectra of several brown diamonds measured at 77 K show a central group of three equally strong lines, with two groups of six closely spaced anisotropic lines of equal intensity at about 30-40 G on either side of the central group. The central group of lines has double the intensity of the side lines. Spacings within the groups are about 5 G. These lines are interpreted as arising from a centre with two substitutional nitrogen atoms in next-neighbour positions. The hyperfine and superhyperfine couplings, which have axial symmetry, are due to a single electron coupling with both nitrogens, and their respective values are A_parallel=413±03 × 10⁻⁴ cm⁻¹, A_{perpendicular}=292±03 × 10⁻⁴ cm⁻¹ and A prime _parallel=52±03 × 10⁻⁴ cm⁻¹, A prime _{perpendicular}=44±03 × 10⁻⁴ cm⁻¹. The centre has an isotropic g of 20028±00002. Measurements of the angular dependence of the lines at 77 K and at higher temperatures showed that the N-C-N centre was not randomly distributed between all possible orientations but confined to sets of parallel {110} planes in the diamond. Above 200 K, averaging effects were observed and, from measurements of the broadening of the lines with increase of temperature when slow hopping was taking place, and the narrowing of new lines that appeared at high temperatures, the activation energy of the electron hopping between the two N-C bonds was found to be 024±001 eV.