R3: High-pressure science and deep Earth’s material

[R3-12] Sound velocity of B2-FeNiSi alloy at high pressure and high temperature and constitution of the Earth’s inner core

*Eiji OHTANI¹, Daijo IKUTA², Hiroshi FUKUI^3,4, Tatsuya SAKAMAKI¹, Daisuke Ishikawa^3,4, Alfred Q. R. BARON^3,4 (1. Tohoku University, 2. Institute for Planetary Materials, Okayama University, 3. JASRI, 4. RIKEN)

Keywords:Sound velocity, B2-FeNiSi alloy, High pressure and high temperature, Inner core

Elastic properties of an ordered derivative of the body-cantered cubic (B2) structure of Fe-7wt% Ni-15wt% Si (Fe_0.67Ni_0.06Si_0.27) alloy have been investigated by combining high-resolution inelastic X-ray scattering and powder X-ray diffraction in diamond anvil cells up to 130 GPa and 2300 K. The density (ρ)-compressional wave velocity (V_P) relation of this phase shows a weak or almost negligible temperature dependence, similar to that observed for the body-centered cubic phase of pure iron (Shibazaki et al, 2016) and the B20 phase of iron-silicon alloys (Whitaker et al., 2009). The ρ, V_P and shear wave velocity (V_S) were extrapolated to the inner core conditions and compared with the PREM (preliminary reference Earth model) inner core (Dziewonski and Anderson, 1981). The phase relation of the Fe-Ni-Si system revealed that B2-Fe-7wt% Ni-15wt% Si alloy coexists with hexagonal close-packed (hcp) Fe-Ni alloys with negligible amount of silicon (Ikuta et al., 2021). The V_P and V_S of the two-phase mixture of B2 and hcp phases under the inner core conditions show slightly higher V_P and V_S compared to the PREM inner core. The two-phase mixture with the addition of a small amount of sulfur could potentially explain the properties of the PREM inner core.