Superstoichiometric Alloying of H and Close‐Packed Fe‐Ni Metal Under High Pressures: Implications for Hydrogen Storage in Planetary Core

Abstract

The Earth's core is primarily composed of Fe alloyed with ∼5.5 wt.% Ni (McDonough, 2003). The observed density deficit suggests a considerable amount of light elements in the core (Birch, 1964). Hydrogen has been considered as one of the light element candidates (Poirier, 1994). Despite its dominance in planetary systems, however, the effects of hydrogen are not well understood because of experimental challenges associated with its study at high pressure-temperature (P-T). Thanks to recent technical advances, such as pulsed laser heating with gated X-ray diffraction (Deemyad et al., 2005;Goncharov et al., 2010), and an evolving vision of the important role of hydrogen in the dynamics and compositions of the interiors of a wide range of planets (Seager & Deming, 2010), the last decade has seen a surge in studies of the Fe-H system under high P-T conditions (

Stability of hydrides in sub-Neptune exoplanets with thick hydrogen-rich atmospheres

“…S3 and S4 A ). As shown in previous high-pressure studies, hydrogen alloys with liquid Fe metal at high - and form FeH , , which is also confirmed here ( 21 , 25 , 26 ) (in our experiments, ; SI Appendix , Fig. S5 ).…”

“…At 8-13 GPa after heating, XRD patterns showed the peaks from Mg 2 FeH 6 , Mg(OH) 2 , face-centered cubic (fcc) FeH x , and MgO (Figs 2a, S3, and S4a). As shown in previous high-pressure studies, hydrogen alloys with liquid Fe metal at high P -T and form FeH x , Fe + 0.5xH 2 → FeH x , which is also confirmed here (21,25,26) (in our experiments, x = 1.0−1.2; Fig. S5).…”

“…In recent studies, possible chemical reactions between hydrogen and silicates/metals have been drawn interest for their potential impacts on planetary processes ( 11 , 12 , 21 ). A high-pressure study ( 11 ) showed that H can induce the reduction of Fe in (Mg,Fe)O to Fe metal.…”

“…However, models so far considered mostly physical mixing of H (or H ) in silicate magma ( 9 , 46 ). Latest experiments begin to reveal other important interactions between hydrogen-rich atmosphere and magma: H O formation through the Fe reduction ( 11 ), and H alloying with Fe metal ( 21 ). Here, we demonstrated that anionic hydrogen, H (or hydride), can exist in the setting expected for sub-Neptune exoplanets.…”

“…A double-sided laser-heating setup was combined with X-ray diffraction for measurements at high - ( 18 ). Because of the diamond embrittlement by H, we conducted pulsed laser heating ( 19 – 21 ) ( SI Appendix , Text 2 ) to reach temperatures close to the melting of MgO. While the pulse heating method enables us to reach temperatures above 2,000 K for the H-rich sample, the heating duration is 0.1 to 0.2 s, which is not sufficient for measuring X-ray diffraction patterns with low noise.…”