New Insights into Gold's Atomic Structure Under Extreme Pressure

Researchers at Lawrence Livermore National Laboratory (LLNL), in collaboration with other institutions, have achieved a significant advancement in understanding gold's atomic structure under extreme pressures. By compressing gold to approximately 10 million times Earth's atmospheric pressure, the team conducted the highest-pressure structural measurement ever recorded for the material. This groundbreaking study sheds light on how matter behaves in the high-pressure environments found in giant planets and fusion research.

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The experiments utilized tailored laser pulses from the National Ignition Facility (NIF) and the OMEGA EP Laser System at the University of Rochester. These facilities enabled the researchers to compress gold samples rapidly while maintaining lower temperatures than typical high-pressure experiments, allowing the gold to remain solid. The team employed X-ray diffraction to capture atomic-scale snapshots within a billionth of a second, providing unprecedented insight into gold's atomic rearrangements under extreme conditions.

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Under normal conditions, gold atoms are arranged in a face-centered cubic (FCC) structure, which is stable across a wide range of pressures. However, the new measurements indicate that this FCC structure persists to much higher pressures than previously predicted, extending to about twice the pressure found at Earth's core. At even higher pressures, the researchers observed the emergence of a body-centered cubic (BCC) structure, where atoms occupy the corners of a cube with one atom at the center. Notably, the original FCC pattern did not vanish entirely; instead, both structures were found to coexist under the extreme compression achieved in the study.

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The findings have significant implications for various scientific fields. Gold is commonly used as a pressure calibrant due to its chemical stability and detectability through X-ray analysis. Understanding gold's behavior at extreme pressures is crucial, as discrepancies in previous models have impacted the accuracy of experiments relying on gold as a reference material. Amy Coleman, a scientist at LLNL and study author, emphasized that "knowing precisely how gold behaves ensures that every other experiment using it as a calibrant... is grounded in a robust and validated understanding of gold’s behavior."

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The study, published in *Physical Review Letters*, not only resolves long-standing inconsistencies between theoretical predictions and experimental observations but also highlights the necessity for precise temperature diagnostics at high pressures. This enhanced understanding of gold's structural transitions lays a stronger foundation for future high-pressure research, bolstering confidence in experiments that explore conditions within planetary interiors and high-energy physics.

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• “These experiments uncover the atomic rearrangements that occur at some of the most extreme pressures achievable in laboratory experiments,” — Amy Coleman, LLNL Scientist

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• “These experiments extend structural measurements of gold into the terapascal regime and highlight the need for temperature diagnostics to refine phase boundaries,” — Amy Coleman, LLNL Scientist

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• “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,” — Amy Coleman, LLNL Scientist

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