A paper led by our alumnus Dr. Mori on the volume expansion of hcp iron hydride has been published in Journal of Geophysical Research: Solid Earth.
Hydrogen is one of the leading candidates for a light element in planetary cores. When hydrogen is incorporated into iron, it causes the volume expansion of iron. In this study, we conducted in situ X-ray diffraction experiments at 10–25 GPa and 300–900 K on hexagonal close-packed iron hydride (hcp-FeHx) synthesized with a controlled hydrogen content, examining its volume changes under high-pressure and high-temperature conditions. We found that the volume expansion per hydrogen atom decreases with increasing pressure and that the effect of temperature becomes weaker at higher pressures. We also showed that the hydrogen-induced volume expansion depends on the crystal structure of iron: the hcp iron requires less hydrogen than the fcc iron to achieve the same density reduction. These findings demonstrate that pressure, temperature, and crystal structure must be properly considered when estimating the hydrogen content of planetary cores from their densities.
Mori Y., Takano M., Kagi H., Aoki K., Kakizawa S., Tsujino N., and Higo Y. (2026) Hydrogen-induced volume expansion in hexagonal close-packed iron: Effects of pressure, temperature, and structure. Journal of Geophysical Research: Solid Earth, 131, 9 e2026JB034491