PHASE DIAGRAMS OF FEO AND FE-SI ALLOYS

Earth's core is less dense than pure iron, implying the presence of one or more lighter element(s) such as Si, O, or S. The phase diagrams of iron alloyed with these elements at high pressures and temperatures (P-T) are critical input for understanding the thermodynamics of these systems, crystallization of the inner core, and the thermal structure of the Earth. Here we present results on FeO and a suite of Fe-Si alloys.

High P-T conditions (up to 200 GPa) were generated using a laser-heated diamond anvil cell. In situ X-ray diffraction to determine crystal structures was performed at beamline 13-ID-D of the Advanced Photon Source, Argonne National Laboratory. Melting was determined from diffuse X-ray scattering, by laser power-temperature relationships, and by temperature-emissivity relationships.

We have determined the melting curve of FeO and clarified the location and slope of the B1/B8 phase transition. We also identified an insulator-metal transition. The B1 metallic phase of FeO is the stable phase at conditions of Earth's lower mantle and outer core, with possible implications for the high P-T character of Fe-O bonds, magnetic field propagation, and lower mantle conductivity.

FeSi has the B20 structure at 1 bar, the B2 structure at high pressures, and a wide two-phase field in between. Fe-9Si has the hcp structure at high P and low T, and converts to an hcp+B2 mixture and then fcc+B2 with increasing temperature. Fe-16Si has the D0₃ structure at low pressures and is an hcp+B2 mixture at higher pressures. We have also measured melting temperatures for each alloy. Phase diagrams in P-X and T-X space imply that the stable phase of Fe-Si alloy at inner core conditions for compositions that match the observed density deficit is an hcp+B2 mixture.