Influence of annealing treatment on as-grown Ib-type diamond crystal at a high temperature and high pressure

In this paper, we report on the influence of annealing treatment on as-grown Ib-type diamond crystal under high pressure and high temperature in a china-type cubic anvil high-pressure apparatus. Experiments are carried out at a pressure of 7.0 GPa and temperatures ranging from 1700 ℃ to 1900 ℃ for 1 h. Annealing treatment of the diamond crystal shows that the aggregation rate constant of nitrogen atoms in the as-grown Ib-type diamond crystal strongly depends on diamond morphology and annealing temperature. The aggregation rate constant of nitrogen remarkably increases with the increase of annealing temperature and its value in octahedral diamond is much higher than that in cubic diamond annealed at the same temperature. The colour of octahedral diamond crystal is obviously reduced from yellow to nearly colorless after annealing treatment for 1 h at 1900 ℃, which is induced by nitrogen aggregation in a diamond lattice. The extent of nitrogen aggregation in an annealed diamond could approach approximately 98% indicated from the infrared absorption spectra. The micro-Raman spectrum reveals that the annealing treatment can improve the crystalline quality of Ib-type diamond characterized by a half width at full maximum at first order Raman peak, and therefore the annealed diamond crystals exhibit nearly the same properties as the natural IaA-type diamond stones of high quality in the Raman measurements.