| The alloying effects of silicon and nickel on the phase diagram and physical properties of iron were investigated under high pressures and high temperatures. Adding silicon into iron stabilizes the bcc phase to much higher pressures and temperatures. Earth's inner core may be composed of hcp-Fe with up to 4 wt% Si, but it is also conceivable that the inner core could be a mixture of a Si-rich bcc phase and a Si-poor hcp phase. The substitution of silicon into iron would lower the density of iron but does not significantly change its compressibility in the bcc phase, nor in the hcp phase at high pressures and high temperatures. Upon comparison with the Preliminary Reference Earth Model (PREM), the calculated equations of state (EOS) of hcp-Fe7.9wt%Si, using the Mie-Grüneisen EOS, indicate that an outer core containing about 8–10 wt% Si and inner core containing about 4 wt% Si in iron would satisfy the density deficit of the core. A small amount of other light element(s) in addition to silicon may also be needed to compensate the density deficit of the outer core. Addition of Si also increases the bulk sound velocity of iron, consistent with Si being a light element in the core.; Adding nickel into iron stabilizes the fcc phase to higher pressures and lower temperatures, and a region of two-phase coexistence between fcc and hcp phases is observed. Iron with up to 10 wt% nickel is likely to be in the hcp structure under inner core conditions. The axial ratio (c/a) of hcp-Fe10wt%Ni has a weak pressure dependence, but it increases substantially with increasing temperature. The extrapolated c/a ratio at ∼5700 K and ∼86 GPa is approximately 1.64, lower than a theoretically predicted value of nearly 1.7 for hcp-Fe at 5700 K and inner-core pressure. A lower c/a ratio should reduce the anisotropy of the hcp phase, and hence, more Fe-Ni crystals in the inner core may need to be preferentially aligned in order to explain the seismic wave anisotropy of the inner core. |
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