Study on the characteristics of Ib diamond crystals synthesized with Fe3O4 doped in an Fe–Ni–C system

Abstract

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.