First experimental evidence of superionic matter in Earth's core

A research team including Doctoral Students Yoshihiro Nagaya and Yusuke Okazaki and Professor Kenji Ohta from the Department of Earth and Planetary Sciences at the School of Science at the Institute of Science Tokyo, and Lecturer Haruhiko Dekura from the Geodynamics Research Center at the Advanced Research Institute at Ehime University, has confirmed for the first time world-wide experimental indications suggesting that face-centered cubic iron hydride (fcc FeH_X), a type of iron-light element alloy, enters a superionic state under high-temperature and high-pressure conditions. The study was published in Nature Geoscience.

Based on observational data such as density and seismic wave propagation speeds, the composition of the inner core at the center of the Earth is estimated to be an alloy containing light elements such as hydrogen, oxygen, and carbon in addition to iron and nickel. However, the elasticity of the inner core derived from seismic wave observations does not match the elasticity of iron-light element alloys determined through experiments and theoretical calculations. The precise reasons remain unknown.

Recent theoretical calculations suggest that the superionic state of iron-light element alloys plays an important role as the cause of this unique elasticity in the inner core. In the case of iron-light element alloys, the iron lattice remains a normal solid while only the light elements diffuse at high speeds. Because the superionic state of iron-light element alloys exists only under ultrahigh-pressure and ultrahigh-temperature conditions, it had never been observed experimentally until now.

The joint research team synthesized samples of fcc FeH_X inside a diamond anvil cell and conducted X-ray diffraction measurements on the samples under high-temperature and high-pressure conditions. First, they varied the temperature while maintaining constant pressure to investigate the temperature dependence of the apparent volume of hydrogen (ΔVH), which is evaluated from the iron lattice volume. As a result, they observed a λ-type thermal expansion anomaly in which ΔVH temporarily increases between approximately 1,500 and 1,800 K. This phenomenon is also observed during superionic phase transitions in other materials.

Furthermore, when the temperature was increased under constant pressure while applying a voltage to fcc FeH_X, the hydrogen distribution within the sample changed abruptly around 1,600 K. When X-ray diffraction mapping was conducted after rapid cooling, it was confirmed that hydrogen concentrated on the anode side and became depleted on the cathode side. This indicates that hydrogen moves at high speed in response to an electric field, serving as crucial supporting evidence that the iron hydride alloy enters a superionic state.

The hydrogen mobility under inner-core conditions estimated from these experimental results was found to be lower than the values predicted by theoretical calculations. When calculated from the experimental values, achieving a thermodynamically optimized state for hydrogen in the inner core would require a timescale significantly exceeding the age of the Earth.

This result suggests that even if the iron hydride alloy reaches a superionic state, long-distance hydrogen diffusion across the entire Earth's inner core hardly progresses on the timescale of Earth's history. According to research on elemental abundances in the Sun, meteorite compositions, and early Earth element partitioning models, the Earth's inner core is thought to contain various light elements other than hydrogen, such as carbon, oxygen, silicon, and sulfur.

Therefore, to answer the question of "whether the inner core is in a superionic state," high-temperature and high-pressure experiments must also be conducted for these other light elements. There is a need to apply the methodology adopted here, which focuses on thermal expansion coefficients and light element mobility to confirm and quantify phase transitions of iron-light element alloys into the superionic state, to other iron-light element alloys.

Journal Information
Publication: Nature Geoscience
Title: Experimental indications of superionic behaviour in iron hydride under Earth's core conditions
DOI: 10.1038/s41561-026-02001-5