[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-04] Sound velocity measurement of B2 Fe-Ni-Si alloys at high pressure and high temperature

*Daijo Ikuta^1,2, Eiji Ohtani², Hiroshi Fukui^3,4, Tatsuya Sakamaki², Izumi Mashino¹, Daisuke Ishikawa^3,4, Alfred Q. R. Baron^3,4 (1.Institute for Planetary Materials, Okayama University, 2.Department of Earth Science, Tohoku University, 3.JASRI, 4.RIKEN)

Keywords:Sound velocity, B2 phase, Fe-Ni-Si alloy, High-pressure, Inelastic x-ray scattering

The most reliable knowledge of the Earth's interior is based on the seismic observations. The seismic model, the Preliminary Reference Earth Model (PREM) [1], provides the profiles of sound velocity and density as a function of depth. Comparing the model with high-pressure experiments, pure iron cannot account for the sound velocity and density of the PREM core [2,3]. Geochemical and meteorite studies [4] have suggested that the Earth's inner core consists of iron-nickel alloys with some light elements, the candidates for light elements being sulfur, oxygen, carbon, hydrogen, and silicon. Of these, silicon is one of the most convincing light elements, both from the cosmochemical abundance of silicon and from the studies of chemical reactions in the evolution of the Earth's core. Recent studies show that the two phases of hexagonal close-packed (hcp) and B2 (an ordered derivative of the body-centered cubic structure, known as CsCl type) for iron-nickel-silicon (Fe-Ni-Si) alloys may coexist in the inner core [2]. Therefore, the sound velocity measurement for the Fe-Ni-Si system is important to understand the properties of the Earth's core and to constrain its chemical composition. However, the sound velocity measurement for the Fe-Ni-Si system, especially under high pressure and high temperature conditions, has not been well studied [5].
In this study, we measured the sound velocity of Fe-7wt%Ni-15wt%Si (Fe-7Ni-15Si) alloy with B2 structure at high pressure and high temperature conditions up to 130 GPa and 1700 K using inelastic x-ray scattering (IXS) and x-ray diffraction (XRD) methods at BL43LXU of SPring-8. The IXS intensities were measured by a total of 16 analyzer crystals installed in BL43LXU, and a Soller screen system [6] to reduce noise as much as possible. The sound velocity was derived from the relation between the momentum change and the energy change. The XRD patterns were measured under the same conditions using the flat panel detector, and the density of the sample was calculated. The results show that the relation between sound velocity and density of the Fe-7Ni-15Si alloy in the B2 phase at high pressure and high temperature follows a linear relation, known as Birch's law. We discuss the constraints on the composition of the Earth's core by comparing the sound velocity of the Fe-Ni-Si alloys with that of the PREM.

References:
[1] Dziewonski and Anderson: Phys. Earth Planet. Inter. 25, 297-356 (1981).
[2] Ikuta et al. Commun. Earth Environ. 2, 225 (2021).
[3] Ikuta et al. Nat. Commun. 13, 7211 (2022).
[4] McDonough. Treatise on Geochemistry 2nd Ed. 3, 559-577 (2014).
[5] Dominijanni et al. Geophys. Res. Lett. 49, e2021GL096405 (2022).
[6] Baron et al. AIP Conf. Proc. 2054, 020002 (2019).