Iron hydride shows superionic behavior under Earth's inner-core conditions

Researchers at the Institute of Science Tokyo have obtained experimental evidence that face-centered cubic iron hydride enters a superionic state under pressures and temperatures matching those of Earth's inner core. In a superionic state, the iron lattice remains solid while lighter elements such as hydrogen move through it almost like a liquid. The study, published in Nature Geoscience, was led by doctoral students Yoshihiro Nagaya and Yusuke Okazaki with Professor Kenji Ohta.

The team compressed tiny samples of iron hydride in laser-heated diamond-anvil cells to pressures between 50 and 110 gigapascals and temperatures above 2,000 Kelvin. Using time-resolved synchrotron X-ray diffraction, they tracked changes in the crystal lattice as hydrogen was incorporated. A characteristic lambda-shaped anomaly in the thermal expansion coefficient appeared near 1,590 Kelvin, a signature of phase transitions seen in other superionic materials.

By mapping this transition at different pressures, the researchers identified the boundary between the normal solid and superionic states. Extrapolating that boundary to inner-core pressures showed the predicted transition temperature lies well below the estimated inner-core temperature, indicating iron hydride could exist in a superionic state there.

Additional experiments applying a constant voltage across the sample at high pressure and temperature revealed a sudden change in hydrogen content. After rapid cooling, a uniaxial hydrogen redistribution was observed, confirming high hydrogen mobility in the superionic state. The researchers estimated a hydrogen mobility on the order of 1 µm² J⁻¹ s⁻¹, corresponding to a diffusion coefficient of approximately 10³ µm² s⁻¹.

Despite this increased mobility, the team calculated that hydrogen migration driven by Earth's geomagnetic field would move only about 0.1 micrometers over 10,000 years. At that rate, hydrogen would need more than 100 times Earth's age to travel a distance comparable to the inner core's radius of roughly 1,200 kilometers. This suggests hydrogen incorporated during planetary formation could remain trapped over geological timescales.

The findings provide direct experimental support for a state of matter previously inferred only from molecular dynamics simulations. They may help explain why seismic shear waves travel more slowly through the inner core than expected and could refine models of the core's formation and evolution.