Solid matter acquires a new behavior

Solid matter acquires a new behavior

Exotic magnesium (Mg) structures observed at extreme pressures (more than three times the central pressure of Earth) at the National Ignition Facility support decade-old theories that quantum mechanical forces localize valence electron density (gold) in the spaces between atoms of Mg (gray) to form “electrides”. Credit: Adam Connell/LLNL

Investigating how solid matter behaves at enormous pressures, such as those found in the deep interiors of giant planets, is a great experimental challenge. To help address this challenge, Lawrence Livermore National Laboratory (LLNL) researchers and collaborators took a deep dive into understanding these extreme pressures.

The paper has just been published in The physics of nature with LLNL scientist Martin Gorman as lead author.

“Our results represent a significant experimental advance; we were able to investigate the structural behavior of magnesium (Mg) at extreme pressures – more than three times that of the Earth’s core – that were previously only theoretically accessible,” said Gorman. “Our observations confirm theoretical predictions for Mg and demonstrate how TPa pressures—10 million times atmospheric pressure—force materials to adopt fundamentally new chemical and structural behaviors.”

Gorman said modern computational methods suggested that core electrons bound to neighboring atoms begin to interact at extreme pressures, causing the conventional rules of chemical bonding to break down and the crystal structure to form.

“Perhaps the most striking theoretical prediction is the formation of high-pressure ‘electrides’ in elemental metals, where free electrons in the valence band are packed into localized states in the interionic voids to form pseudo-ionic configurations,” said he. “But reaching the necessary pressures, often above 1 TPa, is experimentally very difficult.”

Gorman explained the work by describing the best way to arrange the balls in a barrel. Conventional wisdom suggests that atoms under pressure, like balls in a barrel, should prefer to stack as efficiently as possible.

“To fit the maximum number of balls in a barrel, they need to be stacked as efficiently as possible, such as a hexagonal or cubic close packing pattern,” Gorman said. “But even the closest packings are only 74% efficient, and 26% is still empty space, so by including smaller balls of the correct size, more efficient ball packing can be achieved.

“What our findings suggest is that, under immense pressure, the valence electrons, which are normally free to move in Mg metal, become localized in the empty spaces between atoms and thus form a nearly massless, negatively charged ion,” he said. “Now, there are balls of two different sizes—positively charged Mg ions and negatively charged localized valence electrons—which means Mg can pack more efficiently and so such ‘electride’ structures become energetically favorable to of the tight packing.”

The work described in the paper required six days of firing at the National Ignition Facility (NIF) between 2017 and 2019. Members of an international collaboration traveled to LLNL to observe the firing cycle and assist with data analysis in the days following each experiment.

State-of-the-art high-power laser experiments on NIF coupled with nanosecond X-ray diffraction techniques provide the first experimental evidence—in any material—that electride structures form above 1 TPa.

“We compressed elemental Mg through the ramp, maintaining the solid state up to peak pressures of 1.32 TPa (more than three times the pressure at the center of the Earth), and observed the transformation of Mg into four new crystal structures,” Gorman said. “The structures formed are open and have inefficient atomic packing, which contradicts our traditional understanding that spherical atoms in crystals should pack more efficiently with increasing compression.”

However, it is precisely this inefficiency of atomic packing that stabilizes these open structures at extreme pressures, as the empty space is needed to better accommodate the localized valence electrons. The direct observation of open structures in Mg is the first experimental evidence of how valence-core and core-core electron interactions can influence material structures at TPa pressures. The transformation observed between 0.96-1.32 TPa is the highest pressure structural phase transition observed so far in any material and the first at TPa pressures, according to the researchers.

Gorman said these types of experiments can currently only be performed at NIF and open the door to new areas of research.

As Much Pressure as the Core of Uranus: First Research Materials and Synthesis Study in the Terapascal Range

More information:
MG Gorman et al, Experimental observation of open structures in elemental magnesium at terapascal pressures, The physics of nature (2022). DOI: 10.1038/s41567-022-01732-7

Provided by Lawrence Livermore National Laboratory

Citation: Under Pressure: Solid Matter Takes On New Behavior (2022, September 20) Retrieved September 20, 2022, from

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