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

[SIT18-04] Effects of nickel on hydrogenation of iron: High-PT neutron diffraction measurements on Fe0.9Ni0.1Dx

Chikara Shito¹, *Hiroyuki Kagi¹, Sho Kakizawa², Katsutoshi Aoki¹, Riko Iizuka¹, Yuichiro Mori¹, Kazuki Komatsu¹, Hiroyuki Saitoh³, Jun Abe⁴, Asami Sano-Furukawa⁵, Takanori Hattori⁵ (1.Geochemical Research Center, Graduate School of Science, University of Tokyo, 2.Graduate School of Advanced Science and Engineering, Hiroshima University, 3.National Institutes for Quantum and Radiological Science and Technology, 4.Comprehensive Research Organization for Science and Society, 5.Japan Atomic Energy Agency)

Keywords:core, iron hydride, neutron diffraction, density deficit, high pressure

Density of the Earth’s inner core is estimated to be about 5% lower than that of Fe_0.9Ni_0.1 alloy. Hydrogen, the most abundant element in our solar system, is one of the most promising light elements contained in the core. We focused on site occupancy of hydrogen and volume expansion induced by incorporation of hydrogen in Fe–Ni–H systems. In the present study, neutron diffraction measurements on Fe_0.9Ni_0.1D_x were carried out at high-PT conditions up to 12 GPa and 1000 K to clarify the effects of nickel on hydrogenation of iron. Crystal structure of Fe_0.9Ni_0.1D_x including site occupancy of hydrogen was refined using the Rietveld method.

Our results indicated that hydrogen-induced volume expansion coefficients v_D of fcc and hcp Fe_0.9Ni_0.1D_x are ~11% and ~33% larger than those of FeD_x, respectively. The maximum hydrogen content of the Earth’s inner core was calculated to be about one to two times the ocean mass. Hydrogen atoms in fcc Fe_0.9Ni_0.1D_x did not occupy tetrahedral sites at all while 10–20% tetrahedral-site occupancy was previously reported in fcc FeD_x. Therefore, the effect of 10% nickel on hydrogenation of iron cannot be ignored in the Earth’s core. However, the P-T conditions in this study were much lower than those of the Earth’s core. It is essential to perform neutron diffraction measurements on the iron–nickel–hydrogen systems at higher P-T conditions to directly reveal the site occupancies of hydrogen and the density reduction by hydrogenation in the Earth’s core.