Lab test supports superionic iron hydride in Earth’s core

A laser strikes a sample held under pressure between small diamond anvils. In this experiment, researchers at the Science Tokyo institute heated iron hydride to above 2,000 kelvins and subjected it to a pressure of up to 110 gigapascals. X-ray diffraction showed an unusual thermal-expansion curve. The Japanese researchers interpreted it as the first experimental evidence that superionic iron hydride can exist in Earth’s core.

Superionic sounds contradictory, but it describes a distinct state of matter: The iron atoms remain bound in a crystal as they do in a solid, while the lighter hydrogen atoms flow through it like a liquid. In Earth’s core, this behavior would be expected under conditions that can only be approximated in the lab: pressures there exceed 300 gigapascals and temperatures exceed 6,000 kelvins. At just 6 gigapascals, graphite turns into diamond.

The key evidence lay in the thermal expansion. The expansion of the iron hydride crystals increased up to 1,590 kelvins; above that point, it fell sharply. The curve resembles an inverted V and matches the pattern shown by other materials as they transition from a solid to a superionic phase. The findings were published in Nature Geoscience.

In a second experiment, the researchers applied an electrical voltage to the crystals. Hydrogen did indeed move through the material in the direction of the voltage, but more slowly than previously calculated. It would take more than a hundred times as long to cross the entire inner core as Earth has existed. The researchers conclude that hydrogen that entered Earth’s core during the planet’s formation will remain trapped there for the foreseeable future.

And what does this change in practical terms? It does not produce a method for obtaining white hydrogen—that is, naturally occurring hydrogen. The measurement does, however, provide a possible piece of the puzzle explaining why seismic secondary waves, or shear waves inside Earth, pass through the core more slowly than expected: Mobile hydrogen could soften the iron alloy and dampen seismic shear.