Hydrogen occupation and hydrogen-induced volume expansion in Fe0.9Ni0.1Dx at high P-T conditions

The density of the Earth's core is several percent lower than that of iron-nickel alloy under conditions of pressure and temperature equivalent to the Earth's core. Hydrogen is one of the most promising constituents accounting for the density deficit, but hydrogen occupation sites and density decrease of iron-nickel alloy caused by hydrogenation have never been investigated. In this study, the phase relation and crystal structure of Fe0.9Ni0.1Hx(D-x) at high pressures and temperatures up to 12 GPa and 1000 K were clarified by in situ X-ray diffraction and neutron diffraction measurements. Under the P-T conditions of the present study, no deuterium atoms occupied tetragonal (T) sites of face-centered cubic (fcc) Fe0.9Ni0.1Dx, although the T-site occupation was previously reported for fcc FeHx(D-x). The deuterium-induced volume expansion per deuterium v(D) was determined to be 2.45(4) and 3.31(6) angstrom(3) for fcc and hcp Fe0.9Ni0.1Dx, respectively. These v(D) values are significantly larger than the corresponding values for FeDx. The v(D) value for fcc Fe0.9Ni0.1Dx slightly increases with increasing temperature. This study suggests that only 10% of nickel in iron drastically changes the behaviors of hydrogen in metal. Assuming that v(D) is constant regardless of pressure, the maximum hydrogen content in the Earth's inner core is estimated to be one to two times the amount of hydrogen in the oceans.

Automated summary

Through in situ X-ray diffraction and neutron diffraction measurements, the phase relation and crystal structure of Fe0.9Ni0.1Hx(D-x) under high pressure and temperature were investigated. It was found that tetragonal sites of face-centered cubic Fe0.9Ni0.1Dx were not occupied by deuterium atoms under the present study conditions, although such occupation was previously reported for face-centered cubic FeHx(D-x). The deuterium-induced volume expansion per deuterium in Fe0.9Ni0.1Dx was larger than that in FeDx, and slightly increased with increasing temperature. This study suggests that a small amount of nickel in iron significantly affects the behavior of hydrogen in metal. Assuming constant v(D) regardless of pressure, the maximum hydrogen content in the Earth's inner core is estimated to be one to two times the amount of hydrogen in the oceans.