Study on the characteristics of Ib diamond crystals synthesized with Fe3O4 doped in an Fe–Ni–C system
Fe3O4 is a common earth mineral, which often exists in the form of inclusions in natural diamond. It is important to explore the effects of various proportions of Fe3O4 dopants on the characteristics of synthetic diamonds as they help us to better understand the formation mechanism of natural O-containing diamonds and the state of Fe3O4 that exists inside the growth environment of natural diamonds. Diamonds were successfully synthesized using a large volume cubic high-pressure apparatus (CHPA) with an FeNi alloy catalyst doped with Fe3O4 under conditions of 6.0–6.5 GPa and 1350–1400 °C. Optical microscopy measurements indicate that as the doping ratio increases, the crystal growth is slower and there is a tendency towards cracking. The XPS spectrum confirmed that oxygen successfully enters the diamond lattice by forming CO and C–O bonds when the amount of Fe3O4 dopants is increased. We noticed that increasing the dopant ratio leads to a gradual decrease in nitrogen content in the crystals under the same pressure; however, with the same dopant, the nitrogen content decreases as the pressure increases. According to Raman spectroscopy, our synthesized crystals are high-quality sp3 structure Ib diamonds, and the full width at half maximum of the crystals gradually increases with increasing dopant concentration. Additionally, the Raman peak position shifts to the long-wave direction at higher dopant ratios. It also implies that, under the same doping conditions, the pressure increase improves the crystallinity ratio in the crystals. X-ray diffraction (XRD) of the inclusion-containing diamonds found that Fe3O4 was reduced to FeO when it was added to the FeNi catalyst.