Particle physics laboratory

Under pressure: solid matter adopts a new behavior

Studying the behavior of solid matter at enormous pressures, such as those found in the inner depths of giant planets, is a great experimental challenge. To help meet this challenge, researchers and collaborators at Lawrence Livermore National Laboratory (LLNL) dove deep into understanding these extreme pressures.

The book has just been published in Natural Physics with LLNL Scientist Martin Gorman as Ith author.

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

Gorman says that modern computational methods have suggested that electrons in the nucleus bound to neighboring atoms begin to interact at extreme pressures, causing conventional rules of chemical bonding and crystal structure formation to break.

“Perhaps the most striking theoretical prediction is the formation of high pressure ‘electrodes’ in elemental metals, where free valence band electrons are compressed into localized states in the empty spaces between ions to form pseudo-ionic configurations,” he said. “But achieving the required pressures, often greater than 1 TPa, is very difficult experimentally.”

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

“To hold the maximum number of bullets in a barrel, they should be stacked as efficiently as possible, like a compact hexagonal or cubic pattern,” Gorman said. “But even the closest wraps are only 74% efficient and 26% is still empty space, so by including smaller bales of the correct size, more efficient bale wrapping can be achieved.

“What our findings suggest is that under immense pressure, the valence electrons, which are normally free to move in the metal Mg, localize into the empty spaces between the atoms and thus form an almost massless ion and negatively charged,” he said. “Now there are balls of two different sizes positively charged Mg ions and negatively charged localized valence electrons meaning that Mg can pack more efficiently and therefore such “electrode” structures become energetically favorable compared to tight packing.

Work carried out at the NIF

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

State-of-the-art high-powered laser experiments on NIF, coupled with nanosecond X-ray diffraction techniques, provide the first experimental evidence in any material of electrature structures forming above 1 TPa.

“We compressed elemental Mg, maintaining the solid state to maximum pressures of 1.32 TPa (more than three times the pressure at the center of the Earth), and observed that the Mg transformed 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, since empty space is needed to better accommodate localized valence electrons. Direct observation of open structures in Mg is the first experimental evidence of how electronic valence-nucleus and core-nucleus interactions can influence material structures at TPa pressures. The transformation observed between 0.96 and 1.32 TPa is the highest pressure structural phase transition ever observed in any material, and the first at TPa pressures, according to the researchers.

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

In addition to Gorman, co-writers include: Amy Lazicki, Marc Cormier, Stanimir Bonev, Richard Briggs, Amy Coleman, Joel Bernier, Federica Coppari, Dayne Fratanduono, Dave Braun, Ray Smith, and Jon Eggert of LLNL; Sabri Elatresh from King Fahd University of Petroleum and Minerals; David McGonegle and Justin Wark of Oxford University; Lisa Peacock and Steve Rothman of the Atomic Weapons Establishment; Roald Hoffmann of Cornell University; Ryan Rygg and Gilbert Collins of the University of Rochester; and Malcolm McMahon from the University of Edinburgh.