Negligible hydrogen solubility in FeS under high-pressure and high-temperature conditions; Implications for hydrogen storage in planetary cores

Takano, Masahiro*; Kagi, Hiroyuki*; Mori, Yuichiro*; Kobayashi, Hiroki*; Aoki, Katsutoshi*; Takahashi, Yoshio*; Ijichi, Yuta*; Kakizawa, Sho*; Tsujino, Noriyoshi*; Higo, Yuji*; Sano, Asami   ; Tsuchiya, Taku*

Iron monosulfide (FeS) is an important sulfide phase in planetary materials and has been proposed as a potential host of hydrogen in planetary interiors. However, hydrogen solubility in FeS at high pressure and temperature remains debated. In this study, hydrogen incorporation in the NiAs-type FeS phase (FeS V) was reexamined using synchrotron X-ray absorption spectroscopy, in situ X-ray and neutron diffraction, and first-principles calculations. The experimental results indicate that the volume expansion observed under hydrogen-saturated conditions is explained by the reduction of Fe to Fe and a decrease in the value of Fe-deficient FeS, rather than hydrogen incorporation into the lattice. These results suggest that hydrogen incorporation in FeS is extremely limited, with an upper bound of approximately 200 ppm up to 30 GPa.