Lab Experiment Finds Iron Hydride Turns 'Superionic' Under Conditions Matching Earth's Inner Core

Earth's inner core, made mostly of iron with small amounts of lighter elements, is predicted to enter an unusual state of matter called the superionic state under extreme pressure and heat: iron atoms stay locked in place while lighter elements such as hydrogen move through the lattice almost like a liquid. Until now, this idea was supported mainly by computer simulations rather than direct experiments.

Researchers at Institute of Science Tokyo compressed samples of iron hydride, an iron-hydrogen alloy, inside a laser-heated diamond-anvil cell to pressures of 50 to 110 gigapascals and temperatures above 2,000 Kelvin, comparable to conditions in Earth's inner core. Using time-resolved X-ray diffraction to track changes in the crystal lattice, the team, led by doctoral students Yoshihiro Nagaya and Yusuke Okazaki with Professor Kenji Ohta, found a distinctive anomaly in the material's thermal expansion near 1,590 Kelvin — a signature of the transition into a superionic state, published in the journal Nature Geoscience.

Extrapolating this transition boundary to the pressure of Earth's inner core showed the predicted transition temperature is well below the core's estimated temperature, suggesting iron hydride could indeed exist in a superionic state deep inside the planet. In a further test, the researchers applied a constant voltage across a sample under high pressure and temperature, then rapidly cooled it, and found evidence that hydrogen had become highly mobile within the superionic state.

Even so, the researchers estimated that hydrogen movement inside the real inner core would remain extremely slow: migration driven by Earth's geomagnetic field would move hydrogen only about 0.1 micrometers over 10,000 years, meaning it would take more than 100 times Earth's age to travel the roughly 1,200-kilometer radius of the inner core. This suggests hydrogen incorporated when Earth formed could remain trapped there over geological timescales, offering clues to help refine models of how the core formed and evolved.