PHASE DIAGRAMS OF FEO AND 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.