Iron hydride adopts exotic matter state under inner-core conditions

Earth’s inner core may be far more dynamic than its solid appearance suggests. New experiments from researchers at the Institute of Science Tokyo have produced strong evidence that iron hydride can enter a “superionic” state under the extreme conditions found deep inside Earth. In this unusual form of matter, iron atoms remain arranged in an organized crystal lattice while hydrogen atoms move through that lattice with liquid-like freedom. The result offers a possible explanation for long-standing mysteries about the inner core, including its unusual seismic behavior and the fate of hydrogen captured during Earth’s formation.

The inner core is composed mainly of iron, but it is believed to contain smaller amounts of lighter elements such as hydrogen, oxygen, carbon, silicon, or sulfur. At pressures and temperatures far beyond anything found naturally at Earth’s surface, these elements can radically change the behavior of iron alloys. Theoretical simulations have suggested that light atoms might become mobile inside an otherwise solid iron structure, creating a superionic material. Until now, however, this prediction had been supported mainly by computer modeling rather than direct laboratory evidence.

The new study focuses on iron hydride, written as FeHₓ, an alloy in which hydrogen occupies sites within an iron lattice. Under inner-core conditions, FeHₓ is expected to adopt either a hexagonal close-packed or face-centered cubic structure, depending partly on its hydrogen content. The Science Tokyo team, led by doctoral researchers Yoshihiro Nagaya and Yusuke Okazaki with Professor Kenji Ohta, investigated the face-centered cubic form. Their findings, published in Nature Geoscience, suggest that this structure undergoes a transition from an ordinary solid to a superionic state at high temperature.

To recreate the environment of Earth’s deep interior, the researchers compressed microscopic samples of iron hydride inside diamond-anvil cells. These devices use two opposing diamonds to squeeze a sample to enormous pressures while still allowing scientists to observe it. The team subjected FeHₓ to pressures ranging from 50 to 110 gigapascals—up to more than one million times atmospheric pressure—and heated it with lasers to temperatures exceeding 2,000 kelvins. Time-resolved synchrotron X-ray diffraction was then used to track the crystal structure while the sample was heated, compressed, and chemically modified.

X-ray diffraction works by measuring how X-rays scatter from regularly spaced atoms. As temperature rises, a crystal generally expands, and the changing distances between its atoms alter the diffraction pattern. In the iron hydride experiments, the researchers calculated changes in lattice volume and hydrogen content from these patterns. They identified a distinctive lambda-shaped anomaly in the thermal expansion coefficient near 1,590 kelvins. Such an anomaly can signal a transition between different physical states, and similar signatures have been observed in other superionic materials.

By repeating the measurements at different pressures, the team mapped the boundary between normal solid FeHₓ and its proposed superionic phase. When this boundary was extrapolated to pressures comparable to those in Earth’s inner core, the predicted transition temperature fell below estimates of the core’s actual temperature. That means iron hydride could plausibly be superionic within the inner core rather than remaining a conventional solid. The researchers emphasize that their measurements provide experimental indications of superionic behavior, rather than a direct microscopic image of hydrogen moving through the lattice.

A second experiment provided a more direct test of hydrogen mobility. The scientists used specially designed, electrically wired diamond-anvil cells to apply a constant voltage across the compressed and laser-heated sample. During the high-pressure, high-temperature experiment, time-resolved X-ray diffraction revealed a sudden change in the hydrogen concentration of FeHₓ. When the sample was rapidly cooled to room temperature, the hydrogen distribution became unbalanced along one direction. This uniaxial redistribution indicated that hydrogen had moved through the iron framework while the material was in its high-temperature state.

From the sample geometry and applied electrical bias, the researchers estimated hydrogen mobility on the order of 1 square micrometer per volt-second, corresponding to a diffusion coefficient of approximately 1,000 square micrometers per second. That value is extraordinarily high compared with the mobility expected for hydrogen locked into a conventional solid lattice. In the superionic state, hydrogen appears to migrate through the iron structure while the iron atoms remain comparatively localized around their lattice positions. This combination of solid-like order and liquid-like atomic transport is the defining feature of superionic matter.

The discovery could also help explain why seismic shear waves move unusually slowly through parts of Earth’s inner core. Shear waves require a material to resist deformation, but the rapid motion of hydrogen may soften the iron alloy’s shear response even while its overall crystal structure remains intact. The result could influence models of the core’s elasticity, composition, and evolution. It may also affect ideas about how the inner core crystallized, how light elements were distributed, and how heat and chemical energy move through the deep Earth.

Despite the high mobility observed in the laboratory, the researchers found that hydrogen would still migrate extremely slowly under the geomagnetic field inside Earth. Their estimate suggests that field-driven hydrogen movement would amount to only about 0.1 micrometer over 10,000 years. At that rate, hydrogen would require more than 100 times the age of Earth to travel a distance comparable to the inner core’s radius, roughly 1,200 kilometers. The implication is striking: hydrogen incorporated into the core during Earth’s formation could remain effectively trapped for geological timescales, even if it exists in a superionic state. The findings open a new window onto the hidden physics of Earth’s center and provide a rare experimental foothold for understanding matter under planetary extremes.

News Publication Date: June 9, 2026

Web References: https://doi.org/10.1038/s41561-026-02001-5

References: Nature Geoscience, “Experimental indications of superionic behaviour in iron hydride under Earth’s core conditions,” DOI: 10.1038/s41561-026-02001-5