Scientists have coaxed one of the universe’s most stubborn elements into a new compound.
Formed under intense pressures, the newly discovered compound packs helium atoms into crystalline iron, researchers report February 25 in Physical Review Letters. The compound joins a short list of materials that incorporate the normally unreactive element and suggests that helium from the early solar system could be stored in the iron that makes up Earth’s core.
Helium is one of the least reactive elements on the periodic table. Like the other noble gases, helium doesn’t gain or lose electrons easily and so does not normally form chemical compounds. But under extremely high pressures, helium can interact with a few other elements, including nitrogen and sodium — and now iron, research shows.
To make the new iron compound, physicist Kei Hirose of the University of Tokyo and his colleagues squeezed iron and helium together in a diamond anvil cell, a high-pressure device that subjected the elements to pressures greater than 50,000 Earth atmospheres and temperatures above 1,000 degrees Celsius. This compression formed crystals containing both iron and helium.
The volume of the crystal formed was larger than that of a crystal of pure iron at the same pressure, the team found. The researchers attributed this increase to helium ions packing into interstitial sites, the tiny spaces between iron atoms in the crystal. But the helium atoms don’t bond directly to iron — they’re too unreactive, even at extreme conditions.
“Helium is very happy as it is. It doesn’t want to share an electron,” says Stefano Racioppi, a chemist at The State University of New York at Buffalo. But it can still participate in “chemistry without chemical bonds” to form these ordered, crystalline compounds.
The new compound could help explain observations of helium in Earth’s interior, Hirose says. Most of Earth’s helium atoms have two neutrons and form from the radioactive decay of elements like uranium. But some ocean volcanic eruptions release helium atoms with just one neutron. These atoms first formed shortly after the Big Bang. Earth picked up this “primordial” helium as the planet formed.
Its loss from magma suggests that the planet has a deep reservoir of primordial helium, and the new compounds suggest that Earth’s iron-rich core could hold some of that helium. But the team will need additional experiments to determine whether helium is more likely to reside in Earth’s core or higher in its mantle.
“The partitioning of helium between magma, silicate melt and metallic iron is really the key,” Hirose says. If helium is more stable in iron than in the silicates found in the mantle, that would suggest that the helium is more likely to reside in the core — and vice versa.
Computational physicist Ronald Cohen agrees. “I wouldn’t say that it’s a proof that there’s helium in Earth’s core, but it suggests that it’s possible,” says Cohen, of the Carnegie Institution for Science in Washington, D.C.
Aside from the geophysical implications, the findings could further expand noble gas chemistry. “I would like to see whether this is unique for iron or if it could also happen to other transition metals,” says Maosheng Miao, a chemist at California State University, Northridge. Forming other helium metal compounds, he says, could lead to “chemistry that we never thought of.”
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