Extraordinary pressures and temperatures lead to extraordinary chemistry. Usually, noble gases are among the least reactive elements. Yet tremendously high heat and pressure can bring out chemical traits never seen before. In a simulated environment mimicking the pressure cooker environment of Earth’s core, researchers observe the first stable compound between argon and a metal: ArNi (ACS Earth Space Chem. 2019, DOI: 10.1021/acsearthspacechem.9b00212).
“At these extreme conditions, argon is not a noble gas anymore,” says Elissaios Stavrou of Lawrence Livermore National Laboratory.
Stavrou’s laboratory is part of small field trying to understand the chemical rules that reign at high heat and pressures. Last year, he found that xenon reacts with iron and nickel at conditions that exist in Earth’s core. This time, he turned to argon. Nickel isn’t known to react with argon under any circumstances, although Stavrou knew other researchers had convinced argon to form transient compounds with a handful of other metals.
Some geochemical models suggest that radioactive potassium is part of Earth’s core, helping heat the planet. When radioactive potassium decays, it produces argon. If argon is in Earth’s core, Stavrou wondered, would it react?
To find out, Stavrou’s group put a small amount of powdered nickel in between two micrometer-scale diamonds in a diamond anvil cell, added argon gas, and squeezed the diamonds together. Once the pressure reached 140 GPa, 1,300 times the pressure at the ocean’s lowest depth, the researchers turned on an infrared laser to heat the mixture. Close to 1,500 K, the temperature of a hot charcoal fire, the X-ray diffraction pattern changed, indicating that argon and nickel had formed something new.
To figure out the material’s structure, Stavrou’s group collaborated with Yansun Yao and Adebayo A. Adeleke of the University of Saskatchewan. Experiments pointed to an alloy of 50% argon and 50% nickel. Theoretical calculations, Adeleke says, matched a crystal structure of an intermetallic alloy, showing argon and nickel equally interspersed in a solid that is metallic in nature.
Stavrou and his colleagues suggest that this reactivity of argon at core conditions opens up the possibility of argon’s presence in Earth’s core and may help solve outstanding geological questions about argon’s location.
Hans Keppler of the University of Bayreuth, who studies the behavior of trace elements under Earth’s surface, finds the new noble gas chemistry interesting. However, he doesn’t think it changes our understanding of the geochemistry of argon. A crystalline phase that forms from pure argon and nickel in the laboratory “does not mean that such a phase would form when argon is an extremely dilute trace component, as in Earth’s interior,” he says.
Yingwei Fei, who examines high temperature and pressure materials at the Carnegie Science Geophysical Laboratory, agrees that “any implication for Earth’s core is speculative.” He is intrigued, however, by this and other recent findings that break down traditional expectations of element behavior at high temperatures and pressure.