Negligible Hydrogen Solubility in FeS under High-Pressure and High-Temperature Conditions: Implications for Hydrogen Storage in Planetary Cores

Iron monosulfide (FeS) is a major sulfide phase in planetary materials and a potential host of hydrogen in planetary interiors. However, the hydrogen solubility in FeS under high-pressure and high-temperature conditions remains controversial. Here we reassess hydrogen incorporation in the NiAs-type phase of FeS (FeS V) using synchrotron X-ray absorption spectroscopy, in situ synchrotron X-ray diffraction, in situ neutron diffraction, and first-principles calculations. Our X-ray absorption and in situ X-ray diffraction results show that the volume expansion observed under hydrogen-saturated conditions is consistent with the reduction of Fe3+ to Fe2+ and a decrease in the δ value of Fe-deficient Fe1−δS rather than hydrogen incorporation into the FeS lattice. Rietveld refinements of the neutron diffraction pattern further show that a hydrogen-free structural model incorporating preferred-orientation and extinction corrections provides the best fit, while unrealistically large atomic displacement parameters are required to achieve a similar level of fitting using the hydride model. Consistently, first-principles calculations predict that plausible FeS hydride configurations are energetically unstable. Based on these results, we estimate an upper limit of hydrogen in the FeS lattice as ∼200 ppm up to 30 GPa. The negligible incorporation of hydrogen in FeS implies preferential partitioning of hydrogen into metallic Fe during the early stages of core formation. Since FeS and Fe3C are hydrogen-incompatible whereas Fe–Ni and Fe–Si alloys can accommodate hydrogen, the influence of hydrogen during the early stages of core formation should be evaluated in terms of its effective concentration in the metallic phase (i.e., the hydrogen-to-metal ratio) rather than its abundance in the bulk core.