Oxygen and iron are the most abundant elements on Earth and their compounds are important planet-forming components. Although oxygen is ubiquitous in the mantle, its presence in the solid inner core is still debatable.

Researchers led by Dr. Yang Sun of Columbia University and Dr. Jin Liu of HPSTAR (the Center for High-Pressure Science & Technology Advanced Research) have found that Fe-rich Fe-O alloys are stable at nearly 300 GPa and temperatures as high as 3,000 K. The findings suggest that oxygen may exist in the solid inner core, providing important limitations for a better understanding of the formation process and evolutionary history of the Earth’s core.

Because the inner core is beyond human reach, we can only guess at the density and chemical composition of the seismic signals that produce earthquakes. Although their type and composition are still under debate, the inner core is now believed to contain light components. Based on cosmochemical and geochemical data, it is predicted to include sulfur, silicon, carbon and hydrogen. Calculations and experiments also supported the finding that under the high temperatures and high pressures of the deep Earth, these elements combine with pure iron to create a variety of Fe alloys.

However, oxygen is usually excluded from the inner core, as Fe-O alloys with iron-rich compositions have never been found in the surface or mantle environments. All known iron oxides have an oxygen concentration greater than or equal to 50% atomic percentage. While attempts have been made to make iron oxide compounds with high iron content compositions, such materials have not yet been discovered. Is the Earth’s core that “anoxic”? In this work, several tests and theoretical calculations were performed to provide an answer to this question.

Pure iron and iron oxide were placed on the tips of two diamond anvils and fired with a high-energy laser beam to a temperature and pressure close to the Earth’s core. After numerous attempts, it was discovered that above 220-260 GPa and 3000 K a chemical reaction takes place between iron and iron oxide. According to the XRD data, the reaction product is distinguished from the typical high temperature and high pressure structure of pure iron and iron oxide.

A theoretical investigation of the crystal structure based on genetic algorithms has shown that the iron-rich Fe-O alloy can survive steadily at a pressure of about 200 GPa. The new Fe-rich Fe-O alloys develop a hexagonal close-packed structure under such conditions, with the oxygen layers placed between the Fe layers to support the structure. A process like this creates many Fe-rich Fe-O compounds with high configurational entropy and close-packed configurations. These theoretical data were used to identify an Fe28O14 atomic arrangement that matched the experimentally recorded XRD pattern.

Further calculations showed that Fe-rich Fe-O phases are metallic, unlike common iron oxides at low pressure. The electronic structure depends on the O2 concentration and the Fe and O layer arrangements. The mechanical properties and thermal properties of the alloy need further study in the future.

Magazine reference:

  1. Jin Liu, Yang Sun et al. Iron-rich Fe-O compounds at Earth’s nuclear pressures. The innovation. DOI: 10.1016/j.xinn.2022.100354