Transformation of Nitrogen State in High-Level N-Doped Gem-Quality Diamond Crystal Annealed at High Temperature and High Pressure

The colourless IaA-type gem-quality diamond crystals containing a high concentration of nitrogen (1500–1700 ppm) were successfully prepared by annealing the as-grown Ib-type N-doped diamonds at a high temperature and high pressure in China-type cubic anvil high-pressure apparatus. Experiments were carried out at pressures of 6.5–7.0 GPa and temperatures from 1900 K to 2100 K. Annealing treatment on high-level N-doped diamond crystals shows that the colour of the diamond crystals is obviously reduced from green to colourless after annealing treatment within 1 h at a higher temperature, which is induced by nitrogen aggregation in the diamond lattice indicated by infrared (IR) spectroscopy. It is further revealed that active energy of the nitrogen atom transforming from the dispersed form to the aggregated form is much lower than that in the standard Ib-type diamond crystals with nitrogen concentration less than 300 ppm. The colourless IaA-type diamond crystal prepared by annealing at 2100 K displays the same properties in IR spectra as the high-quality natural diamonds which are classified into the IaA type.