Phase Diagrams of Fe-Si Alloys under High Pressures

Introduction Iron is the most abundant element in the Earth’s core. However, the density of the outer core is about 10% lower than the density of iron at the pressure and temperature conditions of the outer core, indicating the presence of a low-atomic-weight component (such as H, C, O, Si, or S) in the core [1]. There is also evidence that the inner core may be less dense than pure iron and that the amount of light elements in the inner core may be about 0-3 wt% [2-4]. Silicon may be an important alloying element in the outer core, on the basis of its cosmochemical abundance and its measured thermoelastic properties [5, 6]. Silicon was excluded as the primary alloying element in the outer core on the basis of the equation of state (EOS) of the intermediate compound ε-FeSi [7]. However, studying the iron-rich portion of the Fe-Si system is more appropriate for understanding the possible effect of silicon on the EOS and crystal structure of iron under core conditions. The phase diagram of iron has been extensively studied. Body-centered cubic (bcc) iron transforms to the hexagonal close-packed (hcp) phase under high pressures, and the bcc phase transforms to the face-centered cubic (fcc) phase under high temperatures [8]. In situ x-ray diffraction studies to 161 GPa and 3000K demonstrate that hcp-Fe has a wide stability field extending from the deep mantle to core conditions [9]. We studied the ironrich portion of the Fe-Si alloys in a large volume press (LVP) in order to understand the possible crystal structures and the phase diagram relevant to the Earth’s core.