Matter changes on an atomic level as it goes down below the surface of the Earth or inside the center of the Sun. The mounting pressure within planets and stars can cause metals to become nonconducting insulators. For instance, sodium has been shown to transform from a shiny gray metal into a transparent glass-like insulator when squeezed hard.

Changes Under High Pressure

It has been theorized that high pressure can squeeze sodium's electrons out into the spaces between atoms. Quantum chemical calculations also show that these electrons still belong to the surrounding atoms and are bonded to each other chemically. It was once thought that materials always have the tendency to become metallic under high pressure, such as the metallic hydrogen which is assumed to make up the core of Jupiter.

When exposed in high pressure, electrons become trapped within the interspatial regions between the atoms in a condition known as electride state. This causes sodium to transform physically from shiny metal to transparent insulator as free-flowing electrons absorb and retransmit light, while trapped electrons simply allow the light to pass through.

Chemical Bonding From High-Pressure Transformation

This particular high-pressure phenomenon has been investigated by experts from University at Buffalo. They try to answer why sodium becomes an insulator, while predicting how other elements and chemical compounds behave at extremely high pressures.

The study "On the Electride Nature of Na-hP4" builds upon the theoretical predictions of the late physicist Neil Ashcroft, whose memory the research is dedicated to. It was Ashcroft, along with researcher Jeffrey Neaton, who found that some materials, like sodium, can actually become insulators or semiconductors when squeezed. They assumed that the core electron of sodium, previously known to be inert, would interact with each other and the outer valence electrons when exposed to extreme pressure.

According to lead author Stefano Racioppi, their current research goes beyond the work of Ashcroft and Neaton, as they connect it with chemical concepts of bonding. The pressures found below the crust of the Earth can be difficult to replicate in a laboratory, so the researchers used supercomputers in UB's Center for Computational Research. They ran calculations on how electrons would behave in sodium atoms under extreme pressure.

For the first time, the researchers' calculations demonstrated the emergence of the electride state of sodium's electrons which can be explained through chemical bonding. Due to high pressure, the electrons occupy new orbitals within their respective atoms. Meanwhile, the orbitals overlap with each other to form chemical bonds and cause localized charge concentrations in the interstitial regions.

While earlier studies provided an intuitive theory that electrons are squeezed out of atoms by high pressure, the new calculations showed that electrons are still part of surrounding atoms. As described by Racioppi, they realized that these are not just isolated electrons which decided to leave the atoms. Instead, the electrons are shared between the atoms in a chemical bond, making them quite special. The findings of this study also reveal how the pressure inside stars and planets can rearrange the atomic structure of a material.