PHASE RELATIONS, DENSITY, AND SOUND VELOCITY OF FE-NI-SI ALLOYS AND COMPOSITION OF THE INNER CORE

The phase relations of Fe, Fe₉₀Ni₁₀ and Fe_87.9Ni_4.4Si_7.7 were investigated up to 273 GPa and 4490 K, 250 GPa and 2730 K and 304 GPa and 2780 K, respectively, and the hexagonal close–packed structure was found to be stable in these compounds under the pressure and temperature conditions studied. The axial ratio of c/a of Fe_87.9Ni_4.4Si_7.7 at 300 GPa showed an almost constant value of 1.593–1.596 against temperature. The weak temperature dependency of the axial ratio of hcp Fe–Ni­–Si alloy indicates that the c–axis is still harder than the a–axis at high temperature condition. The alignment of hcp Fe–Ni–Si alloy with the c–axis parallel to the Earth’s rotation axis could account for the observation of the seismic wave anisotropy of the inner core.

The pressure-volume equations of state of hcp Fe_0.83Ni_0.09Si_0.08 and hcp Fe_0.93Si_0.07 have been investigated up to 374 GPa and 252 GPa, respectively. We obtained the density of these alloys at the pressure of the inner core boundary (ICB), 330 GPa at 300 K by fitting the compression data to the third order Birch–Murnaghan equation of state. Using these density values combined with the previous ones and comparing with the density of the PREM inner core, we estimated that Si content of the inner core is in the range from 7.5 to 12 at %, assuming its Ni-content is 4 to 10 at. %.

We have measured the IXS spectra of hcp-Fe_0.83Ni_0.09Si_0.08 alloy in diamond anvil cell to 135 GPa at 300 K. The sound velocity and the density of the alloy follow a liner relation, i.e., the Birch’s law. The V_p-ρ relation for hcp-Fe_0.83Ni_0.09Si_0.08 can be expressed as V_p (m/sec) = 1.50(0.17)ρ(kg/m²) – 6570(1690) of which slope is steeper than that of pure iron. The extrapolation of the P-wave velocity, V_p, to the inner core along the Birch’s law indicates that the P-wave velocity of the alloy is 4.5% faster than that of the inner core observed in the PREM model. The discrepancy between the Fe_0.83Ni_0.09Si_0.08 alloy and that of the PREM inner core may be reduced by taking account of the temperature effect. The PREM P-wave velocity of the inner core is marginally explained by the composition of the inner core estimated by the density constraints.