Search

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(Dx) 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(Dx). The deuterium-induced volume expansion per deuterium vD was determined to be 2.45(4) and 3.31(6) Å3 for fcc and hcp Fe0.9Ni0.1Dx, respectively. These vD values are significantly larger than the corresponding values for FeDx. The vD 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 vD 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.

地球の核の主要成分であるFeには軽金属が固溶していると考えられている。本研究ではそのモデル物質としてFe0.95Si0.05合金の水素化物について、水素吸蔵サイトと水素誘起格子体積膨張量を中性子その場観察により評価した。14.7 GPaでの測定とリートベルト解析の結果、水素は八面体サイトのみを占有し、水素誘起体積膨張量は2.33Å^3と算出された。SiがFeに加わることによって水素誘起体積膨張量はほとんど変化しないことが明らかになった。密度一定の条件では水素とSiの量は反相関の関係にあると結論づけた。

We applied Kawai-type multi-anvil assemblies (MA6-8) for time-of-flight neutron-diffraction experiments to achieve high-pressures and high-temperatures simultaneously. To achieve sufficient signal ...

High pressure hydrous phases with distorted rutile-type structure have attracted much interest as potential water reservoirs in the Earth’s mantle. An in-situ X-ray diffraction study of β-C...

At high temperatures and high pressures, iron reacts with hydrogen to form hydrides. Three hydride phases are present at moderate temperature and pressure ranges below 2000 K and 20 GPa: α, γ, and e′ phases, respectively, with bcc, fcc, and double-hcp (dhcp) structures. Recent reports have described studies of phase transitions at temperatures and pressures that differ greatly from those predicted from the conventional phase diagram for the Fe–H system established in the early 2000s. We conducted in-situ X-ray diffraction experiments to investigate α/e′–γ phase boundaries at temperatures up to 900 K and pressures up to 9.1 GPa. Phase transition temperatures were determined by averaging those observed upon heating and cooling under constant pressure. The α/e′–γ phase boundaries were found at temperatures 150–300 K higher than the conventional phase boundaries. The α–γ–e′ triple point was expected to relocate accordingly.