Hydrogen (H) and silicon (Si) are considered as important light elements for the planetary cores. A large amount of H is able to alloy with pure Fe metal at high pressures. Si can also alloy well with Fe. However, it remains uncertain how much H can alloy with iron silicides and if it alloys how H can alter the crystal structures of Fe-Si alloys at high pressures-temperatures (P-T). We performed experiments on Fe-9Si and Fe-16Si alloys (9 and 16 wt % Si, respectively) in a H medium up to 42.2 GPa and 3000 K in diamond-anvil cells coupled with pulsed laser heating and gated synchrotron x-ray diffraction techniques. We found conversion of the Fe-Si alloys into Fe-rich (fcc and dhcp FeHx), Si-rich (B20 and B2 FeSi), and intermediate (Fe5Si3Hx) phases. The new Fe5Si3Hx phase has a structure similar to the hexagonal Fe5Si3 phase but with expanded volumes, and thus, possible H incorporation. Both the observed volume expansion and the H content estimated by density-functional theory calculations support a significant amount of H with H/Fe ≈ 0.6 in the crystal structure. Because Fe5Si3 is known to break down above ∼1300 K at ∼18 GPa, our results suggest that hydrogen stabilizes the hexagonal structure at high P-T. These results have implications for the crystallization of Fe-rich liquid at the solid-to-liquid boundary of planetary cores and possible existence of chemical heterogeneities in the solid cores.
|Original language||English (US)|
|Journal||Physical Review B|
|State||Published - Mar 1 2022|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics