Colour changes produced in natural brown diamonds by high-pressure, high-temperature treatment

Absorption and luminescence spectra have been measured for natural brown diamonds before and after high-pressure, high-temperature (HPHT) treatment at 1700-1800 degrees C, and after HPHT treatment at 2025 degrees C. The reduction of the brown colour noted in diamonds with low nitrogen concentration is attributed to the annealing of plastic deformation. In nitrogen-containing diamonds the vacancies released during this annealing are trapped to form N-V-N centres. In addition, the photoluminescence band with a zero-phonon line (ZPL) at 2.526 eV (490.7 nm), characteristic of decorated slip traces, is reduced in intensity during this process, confirming that changes are taking place at the slip traces which have resulted from the plastic deformation. At the lower annealing temperatures the N-V-N centres are in the neutral charge state, giving rise to the H3 absorption band, with a ZPL at 2.463 eV (503.2 nm) and resulting in a yellow colour. At the higher annealing temperature some nitrogen aggregates decompose, producing single substitutional nitrogen which is an electrical donor. Consequently some N-V-N centres are in the negative charge state, and give rise to the H2 absorption band with a ZPL at 1.257 eV (986.9 nm). The combination of absorption in the H2 and H3 bands gives the diamonds a yellow-green or green colour. Annealing at the highest temperature, at pressures very close to the diamond/graphite transition, seems to be effective at reducing the non-radiative recombination channels in the diamonds, and, in white light, many specimens exhibit bright green luminescence, produced by absorbed energy being re-emitted in the H3 band. To the eye these processed diamonds appear similar to the very rare, naturally-occurring, 'green transmitters'. However, because the latter have resulted from much lower geological temperatures, acting over much longer periods of time, they contain negligible concentrations of single substitutional nitrogen and consequently have low H2 absorption. (C) 2000 Elsevier Science S.A. All rights reserved.