This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 (FeHX), 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, published in the journal Nature Geoscience, 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 exists only under ultrahigh-pressure and ultrahigh-temperature conditions, it had never previously been observed experimentally. FeHX 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 FeHX 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 FeHX transitions from a normal solid to a superionic state. 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 FeHX could exist in a superionic state inside Earth's inner core.
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