Sodium's high-pressure transformation can tell us about the interiors of stars, planets

Atomic changes occur in matter as it descends sufficiently under the surface of the Earth or into the sun's core.

Metals have the potential to become nonconducting insulators due to the increasing pressure within stars and planets. Squeezing sodium hard enough has been proven to change it from a lustrous, gray metal into a transparent, glass-like insulator.

Now, the chemical bonding behind this specific high-pressure phenomena has been identified, thanks to research performed by the University at Buffalo.

The electrons in sodium are thought to be basically squeezed out into the gaps between atoms by high pressure, however quantum chemical calculations performed by researchers reveal that these electrons are nonetheless chemically connected to the surrounding atoms and remain a part of them.

The co-author of the study, Eva Zurek, Ph.D., is a professor of chemistry in the UB College of Arts and Sciences. "We're answering a very simple question of why sodium becomes an insulator, but predicting how other elements and chemical compounds behave at very high pressures will potentially give insight into bigger-picture questions," says Zurek. How does a star's inside look like? If there are magnetic fields on planets, how are they produced? How do planets and stars evolve, too? This kind of study brings us one step closer to providing answers to these queries."

The work is dedicated to the memory of the late, great physicist Neil Ashcroft, and it both supports and expands upon his theoretical predictions.

Before Ashcroft and Jeffrey Neaton's groundbreaking study two decades ago, it was believed that materials always turned metallic at high pressure—like the metallic hydrogen assumed to make up Jupiter's core. However, they discovered that some materials, like sodium, can actually turn into insulators or semiconductors when squeezed. They postulated that, at tremendous pressure, the supposedly passive core electrons of sodium would interact with one another and the surrounding valence electrons.

"Our work now connects it with chemical concepts of bonding, going beyond the physics picture painted by Ashcroft and Neaton," says Stefano Racioppi, Ph.D., the main author of the UB-led study and postdoctoral researcher in the UB Department of Chemistry.

The researchers used supercomputers in UB's Center for Computational Research to do calculations on how electrons behave in sodium atoms under high pressure since it may be challenging to replicate the pressures seen below Earth's crust in a lab.

An electride state is created when the electrons are trapped in the spaces between the atoms. Because trapped electrons only let light to pass through, free-flowing electrons absorb and retransmit light, causing sodium to physically change from a shining metal to a transparent insulator.

Nevertheless, calculations made by the researchers revealed for the first time that chemical bonding can account for the development of the electride state.

Electrons in their respective atoms take up new orbitals due to the high pressure. Thereafter, these orbitals cross across to create chemical bonds, which result in localized charge concentrations in the interstitial spaces.

The new calculations revealed that the electrons are still a part of the surrounding atoms, contrary to the intuitive assumption put out by earlier research that suggested high pressure forced electrons out of atoms.

It dawned on us that there were more than only solitary electrons that had chosen to depart from the atoms. Rather, in a chemical link, the atoms share the electrons," claims Racioppi. "They're quite special."

Malcolm McMahon and Christian Storm of the University of Edinburgh's School of Physics and Astronomy and Center for Science in Extreme Conditions are among the other contributors.

The institution for Matter under Atomic Pressure, a National Science Foundation institution directed by the University of Rochester that investigates how pressure within stars and planets may reorganize the atomic structure of materials, provided support for this work.

"Obviously it is difficult to conduct experiments that replicate, say, the conditions within the deep atmospheric layers of Jupiter," says Zurek, "but we can use calculations, and in some cases, high-tech lasers, to simulate these kinds of conditions."

Provided by University at Buffalo