High-pressure transformation of sodium can tell us about the interiors of stars and planets

Travel deep enough beneath the Earth’s surface or inside the center of the sun and matter changes at the atomic level.

Increasing pressure inside stars and planets can cause metals to become non-conductive insulators. Sodium has been shown to transform from a shiny, gray-colored metal to a transparent, glass-like insulator when pressed hard enough.

Now, a study led by the University at Buffalo has revealed the chemical bond behind this particular high-pressure phenomenon.

Although it has been theorized that high pressure essentially forces electrons out of sodium into the spaces between atoms, the researchers’ quantum chemistry calculations show that these electrons still largely belong to surrounding atoms and are chemically bonded to each other. others.

“We are answering a very simple question: why sodium becomes an insulator, but predicting how other elements and chemical compounds behave at very high pressures could potentially provide insight into larger questions,” says Eva Zurek, Ph. D., professor. in chemistry in the UB College of Arts and Sciences and co-author of the study, published in Modified chemistry, a journal of the German Chemical Society. “What does the inside of a star look like? How are planets’ magnetic fields generated, if any? And how do stars and planets evolve? This type of research brings us closer to answering these questions.”

The study confirms and builds on the theoretical predictions of the late physicist Neil Ashcroft, whose study is dedicated to memory.

It was once thought that materials always became metallic under high pressure – like the metallic hydrogen thought to make up Jupiter’s core – but Ashcroft and Jeffrey Neaton’s seminal paper twenty years ago discovered that some materials, like sodium , can actually become insulators or semiconductors when squeezed. They hypothesized that the electrons in the sodium core, considered inert, would interact with each other and with the outer valence electrons when subjected to extreme pressure.

“Our work now goes beyond the physics picture painted by Ashcroft and Neaton, connecting it to chemical concepts of bonding,” says lead author of the UB-led study, Stefano Racioppi, Ph.D ., postdoctoral researcher at the UB chemistry department. .

The pressures found beneath the Earth’s crust can be difficult to reproduce in the laboratory. Therefore, using supercomputers from the UB Computer Research Center, the team carried out calculations on the behavior of electrons in sodium atoms when they are under high pressure.

Electrons are trapped in the interspace regions between atoms, known as the electride state. This causes the sodium to physically change from a shiny metal to a transparent insulator, because free electrons absorb and retransmit light, but trapped electrons simply let light through.

However, the researchers’ calculations showed for the first time that the emergence of the electride state could be explained by chemical bonds.

The high pressure causes the electrons to occupy new orbitals within their respective atoms. These orbitals then overlap to form chemical bonds, causing localized charge concentrations in the interstitial regions.

While previous studies proposed an intuitive theory that high pressure pushed electrons out of atoms, the new calculations revealed that the electrons are still part of the surrounding atoms.

“We realized that it’s not just single electrons that decided to leave the atoms. Instead, the electrons are shared between the atoms in a chemical bond,” says Racioppi. “They’re pretty special.”

Other contributors include Malcolm McMahon and Christian Storm from the School of Physics and Astronomy and the Center for Science in Extreme Conditions at the University of Edinburgh.

The work was supported by the Center for Matter at Atomic Pressure, a National Science Foundation center led by the University of Rochester that studies how pressure inside stars and planets can rearrange the atomic structure of materials.

“Obviously, it is difficult to conduct experiments that reproduce, for example, the conditions in the deep atmospheric layers of Jupiter,” explains Zurek, “but we can use calculations and, in some cases, high-precision lasers. technology, to simulate this type of conditions. “.