Earth's inner core, composed primarily of iron with a small percentage of light elements, may enter a superionic state at extreme pressure and temperature, according to experimental results from researchers at Science Tokyo. Using laser-heated and electrically wired diamond-anvil cells, the researchers identified experimental signatures of superionic iron hydride at conditions relevant to Earth’s core. These findings provide new insights into the composition and dynamics of Earth's deep interior.
Under the extreme pressures and temperatures in Earth's inner core, iron alloys containing light elements are predicted to enter an unusual state of matter known as the superionic state. In this state, iron atoms remain localized around their lattice sites, while lighter elements such as hydrogen, oxygen, and carbon move through the lattice almost like a liquid.
In addition to the rapid movement of light elements, the resulting shear softening of the alloy is also important in geoscience, as it may help explain why seismic shear waves travel more slowly through Earth’s inner core. However, this idea has so far been supported mainly by molecular dynamics simulations, with no direct experimental evidence.
Now, researchers from Institute of Science Tokyo (Science Tokyo), Japan, have obtained strong experimental indications suggesting that face-centered cubic (fcc) iron hydride (FeH X ), a type of iron–light-element alloy, enters a superionic state under the high-pressure and high-temperature conditions found in Earth's inner core.
The study, made available online on June 09, 2026, and published in Volume 19, Issue 7 of the journal Nature Geoscience on July 01, 2026, was led by doctoral students Yoshihiro Nagaya and Yusuke Okazaki along with Professor Kenji Ohta from the Department of Earth and Planetary Sciences, Science Tokyo.
"Because the superionic state of iron–light-element alloys exist only under ultrahigh-pressure and ultrahigh-temperature conditions, it had never previously been observed experimentally. FeH X is expected to adopt either a hexagonal close-packed or a fcc structure under inner-core conditions, depending on the hydrogen content," says Ohta.
The researchers used time-resolved synchrotron X-ray diffraction (XRD) measurements to monitor changes in the crystal lattice while subjecting fcc FeH X to high pressures and temperatures. They compressed tiny samples inside a diamond-anvil cell to pressures between 50 and 110 gigapascals and heated them to more than 2,000 Kelvin using lasers. By tracking changes in the crystal lattice, they calculated how the lattice volume changed as hydrogen was incorporated into the iron lattice.
The researchers found a characteristic λ-shaped anomaly in the material's thermal expansion coefficient near 1,590 Kelvin, a signature of phase transitions that has also been observed in other superionic materials. By mapping this transition under different pressures, they identified the boundary at which FeH X becomes superionic from normal solid. Extrapolating this boundary to Earth's inner-core pressure showed that the predicted transition temperature is well below the estimated inner-core temperature, suggesting that FeH X could exist in a superionic state inside Earth's inner core.
The researchers also performed time-resolved XRD measurements under high-temperature and high-pressure conditions while applying a constant voltage across the sample, revealing a sudden change in the hydrogen content of FeH X . After rapidly cooling the samples back to room temperature, a uniaxial hydrogen redistribution was observed, providing evidence that hydrogen became highly mobile in the superionic state. Based on the sample geometry and the applied constant bias, the researchers estimated the hydrogen mobility to be on the order of 1 µm 2 J⁻ 1 s⁻ 1 , corresponding to the diffusion coefficient of approximately 10 3 µm 2 s⁻ 1 .
Although hydrogen became much more mobile upon entering the superionic state, the researchers found that its movement under Earth's inner-core conditions would remain extremely slow. Their estimation showed that hydrogen migration driven by Earth's geomagnetic field would result in only about 0.1 µm of movement over 10,000 years. At this rate, it would take more than 100 times Earth's age for hydrogen to travel a distance comparable to the inner core's radius of about 1,200 km. This suggests that hydrogen incorporated during Earth’s formation could remain trapped over geological timescales.
The findings could improve our understanding of the processes occurring in Earth’s core and refine models of how the core formed and evolved.
"These findings are expected to contribute to elucidating seismic-wave velocity anomalies in Earth's inner core and the evolution of Earth's interior," says Ohta
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About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of “Advancing science and human wellbeing to create value for and with society.”
Reference
Title: Experimental indications of superionic behaviour in iron hydride under Earth’s core conditions
Journal: Nature Geoscience
DOI: https://doi.org/10.1038/s41561-026-02001-5
Nature Geoscience
Experimental study
Not applicable
Experimental indications of superionic behaviour in iron hydride under Earth’s core conditions
1-Jul-2026
The authors declare no competing interest.