Investigating how solid matter behaves at enormous pressures, such as those in the deep interiors of giant planets, is a major experimental challenge. To help meet this challenge, researchers and collaborators at Lawrence Livermore National Laboratory (LLNL) have taken a deep dive into understanding these extreme stresses.
Work has just been published in Nature Physics With LLNL scholar Martin Gorman as lead author.
“Our results represent an important experimental advance; we were able to investigate the structural behavior of magnesium (Mg) at extreme pressures – three times higher than in the Earth’s core – that were previously only theoretically accessible,” Gorman said. “Our observations confirm theoretical predictions for Mg and show how TPa is compressed – 10 million times Atmospheric pressure“Forcing materials to adopt fundamentally new chemical and synthetic behaviors”.
Gorman said that modern computational methods suggested this Core Electrons Bonding with neighboring atoms begins to react at extreme pressures, which leads to the breakdown of the traditional bases of chemical bonding and the formation of the crystal structure.
“Perhaps the most striking theoretical prediction is the formation ofthe pressure Electrodes in primary metals, where the valence band free electrons They are compressed into localized states within the empty spaces between the ions to form pseudo-ionic formations. “But getting to the required pressures, often above 1 TPa, is an empirical challenge.”
Gorman explained the work by describing the best way to arrange the balls in the barrel. Conventional wisdom suggests that atoms under stress, such as balls in a barrel, should prefer stacking as efficiently as possible.
“To fit as many balls into the barrel as possible, they should be stacked as efficiently as possible, such as a close hexagonal or cubic packing pattern,” Gorman said. “But even closer packing is only 74% effective and 26% still empty space, so by properly including smaller sized balls a more efficient ball packing can be achieved.
“What our findings indicate is that under tremendous pressure, valence electrons, which is usually free to move throughout the Mg metal, becomes localized in the spaces between the atoms, thus forming an almost massless, negative charge ion. And the valence electrons are locally negatively charged – which means that Mg can pack more efficiently and thus the ‘electrode’ structures become strongly favorable over the nearby filler.”
The work described in the paper required six days of imaging at the National Ignition Facility (NIF) between 2017 and 2019. Members of an international collaboration traveled to LLNL to observe the shot cycle and help analyze data in the days following each experiment.
The latest high-power laser experiments on NIF, along with nanosecond X-ray diffraction techniques, provide the first experimental evidence – in any material – for electrode structures that form above 1 TPa.
“We spin compacted magnesium, maintaining the solid state up to a peak pressure of 1.32 TPa (more than three times the pressure at the Earth’s center), and observed the transformation of magnesium into four new crystal structures,” Gorman said. “The structures formed are open and have inefficient atomic encapsulation, which goes against our traditional understanding that spherical atoms in crystals should stack more efficiently with increasing pressure.”
However, it is precisely the inefficiency of atomic packing that stabilizes these open structures at extreme pressures, since empty space is required to better accommodate localized valence electrons. Direct observation of open structures in Mg is the first experimental evidence of how electron interactions in the valence core and core can affect physical structures at TPa pressures. The observed transition between 0.96-1.32 TPa is the highest pressure structural phase transition to date 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 the NIF and open the door to new areas of research.
MG Gorman et al, Experimental observation of open structures in elemental magnesium at terapascal pressures, Nature Physics (2022). DOI: 10.1038 / s41567-022-01732-7
Lawrence Livermore National Laboratory
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