Melting Behavior and Chemical Properties of the Iron-Carbon System

One of the most challenging experiments related to the laser heating technique in the diamond anvil cell (DAC) is an unambiguous x-ray based detection of melting by recording high quality diffuse x-ray scattering from molten materials at high pressure. Employing a newly developed, advanced, flat top laser heating system at GSECARS, we were able to perform on-line melting experiments at pressures up to 60 GPa. The capability to maintain the molten sample in the DAC for a relatively long time (at least 60 s) allowed us to collect high quality x-ray scattering data suitable for structure analysis even from low-Z molten materials, such as Si, Ge, Fe, Fe3C, Fe7C3 etc. In this work, we focused on the melting behavior and chemical properties of the iron-carbon system at pressures up to ~170 GPa studied with on-line micro x-ray diffraction in a double sided laser heated DAC at GSECARS (Sector 13, APS). Iron carbides (Fe3C, Fe7C3) were synthesized in- situ in the DAC from various mixtures of Fe and C powders with different atomic ratios. We have found that the chemical reaction between iron and carbon takes place independent of the structure of starting phases of iron (fcc or hcp) and carbon (graphite or diamond). The reaction temperature increased gradually from ~1000 K to ~1700 K as pressure increased from 6 GPa to 155 GPa. The melting temperature of iron carbide was found to be systematically lower than for iron by ~300-400 degrees in the pressure range 20-60 GPa. The experimentally measured structure factor and related pair distribution function of iron carbide melt were analyzed and compared with pure iron data at related pressures. High- pressure, high-temperature stability of iron carbide phases at the relevant Earth's mantle-core conditions and physical/chemical properties of iron-carbon melts provide important constraints on models of the formation of D" layer and interactions at the core mantle boundary. Implications of these results for the composition and structure of the Earth's interior will be discussed.