Structure measurement of liquid Fe-C alloys at high pressure

Abstract:

Iron is a primary component of planetary cores and the cores are believed to contain a certain amount of light elements. Carbon is one of the most plausible light elements in the cores. The dynamic process in the liqiuid core (e.g. dynamo) is closely related to physical properties of liquid iron alloys (density, viscosity, and etc.). Although the physical properties of the liquid iron alloys are considered to correlate highly with those local structures, the knowledge about the correlations between the physical properties and the local structures for the liquid iron alloys is still lacking. In this work, we have carried out the structural measurements for liquid Fe-C alloys up to 7.2 GPa using multi-angle energy-dispersive X-ray diffraction (EDXD) technique with a Paris-Edinburg type large volume press at the Sector 16-BM-B beamline at the Advanced Photon Source.

The collected EDXD data shows that the first peak positions of the structure factor S(Q) of liquid Fe-3.5 wt% C and its reduced pair distribution function G(r) are almost constant below 5 GPa, whereas those change with pressure above 5 GPa (S(Q) increases and G(r) decreases). Since the relative scattering factor of carbon atoms is approximately 50 times smaller than that of iron atoms due to small atomic number of carbon (Boronenkov et al., 2012), we considered that the obtained S(Q) and G(r) are basically related to the Fe-Fe bond in the liquid Fe-C alloy. Therefore, these pressure-dependences indicate that the Fe-Fe bond distance in liquid Fe-3.5 wt% C is constant below 5 GPa and then shortens with increasing pressure at least up to 7.2 GPa. The observed change at 5 GPa is in a good agreement with the observed density jump by Shimoyama et al. (2013) at same pressure, indicating that the density jump could result from shortening of the Fe-Fe bond at about 5 GPa.