Exotic magnesium (Mg) structures observed at extreme pressures (more than three times Earth’s central pressure) at the National Ignition Facility support decade-old theories that quantum mechanical forces localize electron density of valence (gold) in spaces between Mg atoms (grey) to form “electric”. Image by Adam Connell/LLNL.
Investigating how solid matter behaves at enormous pressures, such as those found in the deep interior of giant planets, is a major experimental challenge. To help address this challenge, researchers and collaborators at Lawrence Livermore National Laboratory (LLNL) delved deeper into understanding these extreme pressures.
The work has just been published in Nature Physics 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 higher than in the Earth’s core, which were previously only accessible theoretically,” said Gorman. “Our observations confirm theoretical predictions of Mg and demonstrate how TPa pressures – 10 million times atmospheric pressure – force materials to adopt fundamentally new chemical and structural behaviours.”
Gorman said that modern computational methods have suggested that core electrons attached to neighboring atoms begin to interact at extreme pressures, causing the conventional rules of chemical bonding and crystal structure formation to break down.
“Perhaps the most striking theoretical prediction is the formation of high-pressure ‘electrics’ in elemental metals, where free electrons in the valence band are squeezed into localized states within the interionic voids to form pseudo-ionic configurations “, he said. “But reaching the required pressures, often above 1 TPa, is very difficult experimentally.”
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 must be stacked as efficiently as possible, such as a hexagonal or cubic packing pattern,” Gorman said. “But even the closest packings are only 74% efficient, and 26% is still empty space, so including smaller balls of the right size can achieve more efficient ball packing.
“What our findings suggest is that under immense pressure, the valence electrons, which are normally free to move throughout the Mg metal, become localized in the empty spaces between the atoms and thus form an almost massless ion and negatively charged,” he said. “There are now balls of two different sizes: positively charged Mg ions and negatively charged localized valence electrons, which means that Mg can be packed more efficiently and so these ‘electride’ structures become energetically favorable for close packing.”
Work carried out at the NIF
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 help analyze the data on the days after each experiment.
State-of-the-art high-power laser experiments at the NIF, together with nanosecond X-ray diffraction techniques, provide the first experimental evidence, in any material, of electret structures forming above 1 TPa .
“We compressed elemental Mg in a ramp, maintaining the solid state up to maximum pressures of 1.32 TPa (more than three times the pressure at the center of the Earth) and observed that Mg transformed into four new crystal structures ·lines,” 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 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. According to the researchers, 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.
Gorman said these types of experiments can currently only be conducted at the NIF and open the door to entirely new areas of research.
In addition to Gorman, co-authors 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 of King Fahd University of Petroleum and Minerals; David McGonegle and Justin Wark of the University of Oxford; 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 of the University of Edinburgh.
/ Public communication. This material from the original organization/author(s) may be ad hoc in nature, edited for clarity, style and length. The views and opinions expressed are those of the author(s). See them in full here.