Formation of two crystal modifications of Fe7C3−x at 5.5 GPa

The Fe–C system, which is widely used to grow commercial high-pressure–high-temperature diamond monocrystals, is rather complicated due to the formation of carbides. The carbide Fe 3 C is a normal run product, but the pressure at which Fe 7 C 3 carbide becomes stable is a subject of discussion. This paper demonstrates the synthesis of Fe 7 C 3 carbide and its detailed study using single-crystal and powder X-ray diffraction, as well as electron probe micro-analysis and scanning electron microscopy. The experiments were performed using a multiple-anvil high-pressure apparatus of `split-sphere' (BARS) type at a pressure of 5.5 GPa and a temperature of 1623 K. Our results show that in the Fe–C system, in addition to diamond, a phase that corresponds to the Fe 7 C 3 carbide was synthesized. This means that both carbides (Fe 7 C 3 and Fe 3 C) are stable at 5.5 GPa. Two crystal phases are described, Fe 14 C 6 and Fe 28 C 12− x . Fe 14 C 6 is based on the well known rhombic structure of Fe 7 C 3 , while Fe 28 C 12− x has a different packing order of Fe 6 C polyhedrons. The results obtained in this study should be taken into account when synthesizing and growing diamond at high pressures and temperatures in metal–carbon systems with a high iron content, as well as when conducting experimental studies on the synthesis of diamond directly from carbide.