Planetary cores are composed mainly of Fe with minor elements such as Ni, Si, O, and S. The physical properties of Fe alloys depend on their composition. Changes in c/a ratio, center shifts, and elastic properties of Fe and Fe-Ni alloys were reported previously. However, such properties of Fe light-element alloys have not yet been extensively studied. Si is a plausible candidate as a light element in planetary cores. Therefore, we studied the electronic properties and compressional behavior of Fe-Si alloys with a hexagonal-close-packed (hcp) structure under high pressure using synchrotron Mössbauer spectroscopy (SMS) and X-ray diffraction (XRD). Center shifts (CS) were observed at pressures of 21.4-45.3 GPa for Fe-2.8wt%Si and of 30.9-62.2 GPa for Fe-6.1wt%Si. Some of SMS and XRD measurements were performed under the same conditions using a newly developed system at the BL10XU beamline of SPring-8, which allowed simultaneous characterization of the electron information and crystal structure. Changes in the CS values were observed at 36.9 GPa in Fe-2.8wt%Si and 54.3 GPa in Fe-6.1wt%Si. The ratios of c/a in the hcp structure were measured at pressures of 21.2-49.6 GPa in Fe-2.8wt%Si and 32.9-61.4 GPa in Fe-6.1wt%Si. The c/a ratio changed at pressures of 30-45 GPa in Fe-2.8wt%Si and at 50 GPa in Fe-6.1wt%Si. Changes in the CS and c/a ratio were explained according to the electronic isostructural transition in Fe-Si alloys. In addition, the transition pressure increased with increasing Si content in metallic iron. This finding is significant as changes in seismic wave velocities due to the change in c/a ratio of Fe-Si alloys with an hcp structure might be observed if Venus has a solid inner core.
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