Helium Forms Stable Compounds with Iron at High Pressure and Temperature - Academic News

Academic News

As a noble gas, helium is chemically inert and rarely forms compounds with other elements. Researchers from Taiwan and Japan, however, demonstrated for the first time that helium can react with molten iron under high pressure and temperature to form several stable compounds. This groundbreaking study, published in Physical Review Letters and featured in Physics Magazine, provides compelling evidence to support the long-standing hypothesis that the Earth’s core contains a reservoir of primordial helium-3, an isotope of helium-4.

At the University of Tokyo, a team led by Professor Kei Hirose used a diamond anvil cell to compress iron and helium, heating the sample with a laser to achieve high pressures (up to 54 GPa) and temperatures (up to 2820 K). Through synchrotron X-ray diffraction, the researchers observed that the unit-cell volume of the iron lattice in the sample was significantly larger than that of pure iron under the same pressure–temperature conditions. This expansion in volume was attributed to the incorporation of helium atoms into the iron lattice, a hypothesis later supported by secondary-ion mass spectrometry analysis conducted at Hokkaido University. Meanwhile, at National Central University, Professor Han Hsu employed computational approach to investigate the iron-helium compounds, confirming their dynamical stability and revealing detailed atomic structures, magnetic properties, and bonding characteristics.

The implications of this finding extend beyond the realms of physics and chemistry. Helium-3, a product of the Big Bang, is abundant in the solar nebula but rare on the Earth's surface. A long-standing hypothesis suggests that, as the Earth coalesced from the solar nebula, primordial helium-3 may have been trapped in the Earth's core, which is primarily composed of metallic iron, with pressures exceeding 135 GPa and temperatures above 4,000 K. This new finding provides additional support for that hypothesis, offering valuable insights into the formation of our planet.