Probing the Fe₃C Phase Diagram at Conditions Relevant to Planetary Cores with Dynamic Compression Experiments

The cores of Earth and larger super-Earths are believed to be composed of iron (Fe) alloyed with light elements, including carbon (C). Alloying is expected to strongly affect the properties of Fe, including phase stability and melting. However, a poor understanding of the high-pressure-temperature (P-T) phase diagram limits this understanding. At the OMEGA EP laser, we explored the structure of Fe₃C under dynamic loading using X-ray diffraction. We carried out quasi-isentropic ramp experiments and shock-ramp experiments, compressing along a series of high-temperature isentropes near the Fe-C melt line. Under ramp compression, we observe the Pnma phase of Fe₃C up to 550 GPa and find that the equation of state (EOS) diverges from extrapolations based on static data. In shock-ramp experiments, we observe Pnma Fe₃C at P-T conditions of Earth’s inner core. These results contrast predictions that Fe₃C decomposes into Fe metal and carbide phases of different stoichiometries. Our results also place a bound on the Fe-C melt curve at 300-400 GPa, a significant extension of past data. This study provides new constraints on crystal structure, EOS, melting behavior, and kinetics of phase transitions under extreme conditions with applications to understanding the cores of rocky planets.