The inside of giant planets can reach pressures more than one million times the Earth's atmosphere. As a result of that intense pressure, materials can adopt unexpected structures and properties. Understanding matter in this regime requires experiments that push the limits of physics in the laboratory.
In a recent paper published in Physical Review Letters, researchers at Lawrence Livermore National Laboratory (LLNL) and their collaborators conducted such experiments with gold, achieving the highest-pressure structural measurement ever made for the material. The results, which show gold switching structure at 10 million times the Earth's atmospheric pressure, are essential for planetary modeling and fusion science.
"These experiments uncover the atomic rearrangements that occur at some of the most extreme pressures achievable in laboratory experiments," said LLNL scientist and author Amy Coleman.
Gold is a common reference material for high-pressure science. It is often used to calibrate static measurements of pressure because it is chemically stable and is easy to detect with X-rays. Its behavior at low pressure conditions is relatively well-understood, but there have been some historical discrepancies when it comes to extreme pressures.
"Knowing precisely how gold behaves ensures that every other experiment using it as a calibrant, from studying planetary cores to designing new materials, is grounded in a robust and validated understanding of gold's behavior," said Coleman.
But reaching these pressures is extraordinarily difficult. To obtain their measurements, the authors created tailored laser pulses at the National Ignition Facility (NIF) and the OMEGA EP Laser System at the University of Rochester. Those pulses allowed them to access ultra-high pressures at lower temperatures where the gold is still in a solid state.
The process also required ultra-precise timing, with atomic-scale X-ray diffraction snapshots taken in a billionth of a second.
"Only recently have facilities like NIF had the capability to both create these pressures and to take a snapshot of what happens to atoms inside the sample," said Coleman. "This is the first definitive look at gold's crystal structure under such extreme compression, and it finally resolves long-standing disagreements between theory and experiment."
Under normal conditions, gold atoms arrange themselves in a pattern called a face-centered cubic structure. This lattice has atoms at each corner and in the center of each face of a cube. The scientists found this structure to be stable to much higher pressures than some models predicted. It was the only phase of gold present up to about twice the pressure of the Earth's core.
Beyond that, the gold began to change. Some gold atoms arranged themselves into a body-centered cubic structure, where atoms are located at each corner of a cube and one atom is in the exact center. But some of the original face-centered structure also persisted, providing evidence of a coexistence between the states.
"These experiments extend structural measurements of gold into the terapascal regime and highlight the need for temperature diagnostics to refine phase boundaries," said Coleman. "They provide a stronger foundation for using gold as a high-pressure standard and for exploring matter under extreme conditions."
