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Hexagonal Close-packed Iron Hydride behind the Conventional Phase Diagram
Akihiko Machida1, Hiroyuki Saitoh2, Takanori Hattori3
1Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5148, Japan. machida@spring8.or.jp.
Scientific Reports
|August 25, 2019
Summary
Hexagonal close-packed iron hydride (hcp FeHx) forms under specific temperature and pressure conditions, expanding its known phase diagram. This discovery is crucial for understanding iron-hydrogen systems, including planetary interiors.
Area of Science:
- Materials Science
- Geophysics
- Solid-state Chemistry
Background:
- The hexagonal close-packed (hcp) FeHx phase is not currently in the established Fe-H phase diagram.
- Hcp metallic iron is stable under conditions relevant to Earth's inner core.
Purpose of the Study:
- To investigate the formation and properties of hcp FeHx under varying temperature and pressure.
- To determine the hydrogen site occupancy and volume expansion in hcp FeHx.
Main Methods:
- In situ X-ray and neutron diffraction.
- Experiments conducted at 298–1073 K and 4–7 GPa.
Main Results:
- Hcp FeHx forms for compositions with x < 0.6.
- Hydrogen atoms occupy octahedral interstitial sites randomly and partially.
- Hcp FeHx shows a larger H-induced volume expansion compared to fcc FeHx.
- Hcp FeHx exhibits increasing x with temperature, while fcc FeHx shows decreasing x.
Conclusions:
- The study expands the known Fe-H phase diagram by identifying the hcp FeHx phase.
- Findings provide critical data for future research on Fe-H systems across a wide range of compositions, temperatures, and pressures.
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