Effect of pressure on large size diamond single crystal synthesized by temperature gradient method under low nitrogen condition

Pressure is a vital condition in the synthesis of diamond crystals via the temperature gradient method. Exploring the effect of pressure on the growth of diamond crystals is vital. In this study, we investigated the synthesis of diamond crystals using iron-nickel alloy catalysts and analyzed the properties of the synthesized diamonds in the pressure gradient using a China-type large-volume cubic high-pressure apparatus. The synthesis temperature of 1600 K and pressure range of 5.5-6.5 GPa were used. The results of Raman spectra showed that increasing the pressure would increase the crystallinity of diamond crystals. The FTIR spectra of diamond samples featured peaks at 1332 (N+ ions), 1130, and 1344 cm-1, indicating that with increasing pressure, the concentration of nitrogen atoms and ions in diamonds significantly decreased. The photoluminescence spectra of nitrogen ions in diamond samples displayed a peak at 40-100 ppm. The 692 and 694 nm peaks and the phonon bands of NV-chromophores gradually weakened with increasing pressure, indicating that the 692 and 694 nm peaks are attributable to the NV-color center and phonon displacement energy of 165 meV occurred. The Electron paramagnetic resonance spectra displayed a phonon bands of the NV-color center phonon are expressed in EPR through the g-value of NV-.These test results indicated that the pressure featured a significant effect on the nitrogen concentration and the nitrogen-related defects in diamonds.