In the deepest region of the Earth's mantle, just above the core, much of the material consists of crystals of magnesium oxide. In larger rocky exoplanets called super-Earths, the higher pressures in the lower mantle may drive magnesium oxide to transform into a new high-pressure crystal structure, or a metallic, electrically conductive liquid that generates a magnetic field. And in gas giants, magnesium oxide could exist in the cores themselves.
To understand interiors across planet types, researchers need to know exactly how magnesium oxide responds to extreme pressures and temperatures. But for years, magnesium oxide experiments have produced experimental results that theories cannot explain.
Now, after nearly 12 years of investigation, a study from Lawrence Livermore National Laboratory (LLNL) and Johns Hopkins University has solved that problem. The work, the third is a series, was published in Physical Review X.
"We started with a mystery. Why didn't our measurements agree with the expected behavior of magnesium oxide?" said Raymond Smith, LLNL physicist and lead author. "The breakthrough came when we questioned an assumption we had taken for granted: that at pressures approaching twice those at Earth's center and temperatures above 10,000 Kelvin, the direction in which you compress the crystal should no longer matter."
Magnesium oxide's atoms form a cubic crystal structure. To get the material to the extreme conditions that exist in planetary interiors, researchers hit it with a powerful laser pulse that launches a shockwave through the crystal. Until this study, the scientific community generally assumed that the direction of the cube's compression was unimportant to the overall material response at extreme levels of pressure and temperature.
Testing that expectation required an ambitious experimental campaign at the University of Rochester's Laboratory for Laser Energetics, using the Omega EP laser facility. The team launched shocks along three directions: perpendicular to a cube face, along a face diagonal and along the diagonal through the cube.
"What we observed baffled us," Smith said. "None of us expected such a complex response from changing the direction of compression. It took several years, and advances in every part of our analysis, to understand what the measurements were telling us."
The researchers combined simultaneous optical measurements of shock speed and temperature with X-ray diffraction, which shows the arrangement of atoms. Together, the measurements revealed that magnesium oxide's transformation to a denser crystal structure depends strongly on the direction of compression.
"The crystal's orientation determines how the applied compression can drive the atoms into a new arrangement," said LLNL scientist and author Saransh Soderlind. "The diffraction patterns helped us identify those different pathways. When we combined them with the temperature measurements, the pieces finally began to fit together."
Along the diagonal through the cube, the magnesium oxide transformed as expected, transitioning between ordered structures and melting with increasing applied pressure. Along the other directions, the evidence pointed to a more complicated process. A thin, short-lived disordered or liquid region at the front of the shockwave preceded a trail of irregular crystal growth behind it.
The differences can explain earlier experiments that puzzled researchers.
"By connecting the optical measurements of the very thin region at the shock front with the X-rays that probe a much larger region behind it, we were able to truly understand how the direction of compression makes a difference," said LLNL scientist and author Marius Millot. "These insights allowed to explain previous discrepancies between theory and experimental observations."
Despite its different transformation pathways, magnesium oxide consistently melted at approximately 572 gigapascals and 13,200 Kelvin. This is the highest experimentally recorded melting temperature among known materials.
The resulting data provides a benchmark for models of planetary interiors.
"Such high melting conditions could change our understanding of physical processes that shaped the formation of the Earth," said Smith. "Magnesium oxide could crystallize first out of the magma oceans of the early-Earth and also affect the amount of vaporized material from a giant impact."
Extrapolation to higher pressures suggests that crystalline magnesium oxide could persist in parts of Jupiter's and Saturn's cores, while hotter giant planets could contain it as a liquid.
"Magnesium oxide is an important ingredient in our understanding of planetary interiors," said June Wicks, a professor in Johns Hopkins' Department of Earth and Planetary Sciences. "These experiments help us determine where it can remain solid and where it becomes liquid, which matters for how planets transport heat and evolve."
This research illuminates magnesium oxide behavior under extreme conditions, and the findings can be extended to better understand other materials, such as those relevant to stockpile modernization and LLNL's national security mission.
Other members of the team included LLNL scientists Jon Eggert and Federica Coppari and Zixuan Ye, from Johns Hopkins University, who participated as part of her doctoral research.
The research received support from the U.S. Department of Energy's National Laser Users' Facility Program through an award to Wicks at Johns Hopkins, LLNL's Laboratory Directed Research and Development Program and the Defense Threat Reduction Agency. The team also acknowledged the Omega EP operations staff and LLNL's target engineering team.
