In this study, {111}-oriented diamond crystals with different nitrogen concentrations were successfully synthesized in a series of experiments at 5.8 GPa pressure and 1380–1400 °C temperature.

In this study, type Ib diamond annealing experiments were successfully performed under a pressure of 2.5 GPa and a high temperature range between 1680 and 2060 °C. The color of the diamond changed from yellow to light yellow, and the nitrogen (N) state changed from the isolate C-center to the aggregated A-center as the annealing temperature increased. The NV0 center was detected when the annealing temperature was under 1840 °C, and not detected when the temperature reached 1920 °C. The NV– center was more stable than the NV0 center at an annealing temperature of 1920 °C. When the annealing temperature reached 1990 °C, the NE8 center appeared in the diamond lattice. When the annealing pressure changed from 2.5 to 5 GPa, high pressure would restrict the formation of A-center N but hardly affected the formation of NV− center  in the diamond lattice. This was the first known report on the successful preparation of the type IaA diamond under a lower pressure of 2.5 GPa. Our experiment results could be helpful for further understanding the formation of various centers in the diamond lattice and provided data for distinguishing the annealed synthesized diamond from the natural diamond in the jewelry market.

In the NiMnCo–C system, diamond single crystals have been successfully synthesized by adding C3H5N3O as an organic additive under 5.5–6.2 GPa and 1280–1320 °C.