The high-pressure transition of sodium can tell us about the interiors of stars and planets

Travel deep enough below the surface of the Earth or into the core of the Sun, and matter changes at the atomic level.

Pressure building up inside stars and planets can turn metals into non-conducting conductors. Sodium has been shown to change from a shiny, gray metal to a transparent, glassy insulator.

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

Although the high pressure is thought to squeeze sodium's electrons into the spaces between atoms, the researchers' quantum chemical calculations show that these electrons still belong to surrounding atoms and are chemically bonded to each other.

„We are answering the very simple question of why sodium becomes an insulator, but predicting how other elements and chemical compounds behave at very high pressures can provide insight into bigger picture questions,” says Professor Eva Jurek. Associate Professor of Chemistry and Research at UP College of Arts and Sciences, published Applied Chemistry, Journal of the German Chemical Society. „What does a star's interior look like? How do planets' magnetic fields form, if indeed there are any? And how do stars and planets form? Research like this moves us closer to answering these questions.”

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

It was once thought that materials such as metallic hydrogen, theorized to make up Jupiter's core, would always turn metallic under high pressure — but a seminal paper by Ashcroft and Jeffrey Neaton two decades ago found that some materials, such as sodium, actually become conductors or semiconductors when pressed. They hypothesized that sodium's core electrons, considered inactive, interact with each other and with the outer valence electrons when under extreme stress.

„Our work now goes beyond the physical picture painted by Ashcroft and Newton and connects it to chemical concepts of bonding,” says Stefano Racioppi, Ph.D., a postdoctoral researcher in the UP Department of Chemistry and lead author of the UP-led study. .

The pressures found below Earth's crust are difficult to replicate in the laboratory, so using supercomputers at UB's Center for Computational Research, the team ran calculations on how electrons behave in sodium atoms when under high pressure.

Electrons are trapped in the interstices between atoms, which is called an electret state. This causes sodium's physical transformation from a shiny metal to a transparent insulator, as free-flowing electrons absorb and re-transmit light, but trapped electrons allow light to pass through.

However, the researchers' calculations showed for the first time that the appearance of the electrode state could be explained by chemical bonding.

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

While previous studies offered an intuitive theory that high pressure squeezes electrons out of atoms, the new calculations found that the electrons are still part of the surrounding atoms.

„We realized that these weren't isolated electrons that decided to leave the atoms. Rather, the electrons were shared between atoms in a chemical bond,” says Racioppi. „They are very special.”

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

The work was supported by the Center for Matter under Nuclear 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 matter.

„Obviously it's difficult to conduct experiments that mimic the conditions inside Jupiter's deep atmospheric layers, but we can use calculations and in some cases high-tech lasers to simulate these types of conditions.”