Gold is an extremely important material for high-pressure testing and is considered the "gold standard" for pressure calculations in static anvil cell experiments. With slow compression at room temperature (on the order of seconds to minutes), gold is preferably the cubic face-centered structure (fcc) at pressures up to three times the center of the earth.
Researchers at the Lawrence Livermore National Laboratory (LLNL) and the Carnegie Institution of Washington, however, have found that with rapid compression of gold over nanoseconds (one billionth of a second) the pressure and temperature rise changes the crystal structure into a new phase of gold , This known cubic body-centered (bcc) structure turns into a more open crystalline structure than the fcc structure. These results were recently published in Physical Review Letters .
"We have discovered a new structure in gold that exists in extreme conditions – two-thirds of the pressure that prevails in the center of the earth," said lead author Richard Briggs, a postdoctoral fellow at LLNL. "The new structure has a less efficient packing than the initial structure at higher pressures, which was surprising given the variety of theoretical predictions that suggested denser structures."
The experiments were performed on Dynamic Compression Sector (DCS) at the Advanced Photon Source, Argonne National Laboratory. DCS is the first synchrotron X-ray equipment dedicated to dynamic compression science. These user experiments were some of the first to be carried out with hutch-C, DCS's special high-energy laser station. Gold was the ideal object of study due to its high z-value (which provides a strong x-ray scattering signal) and the relatively unexplored phase diagram at high temperatures.
The team found that the structure of gold began to change at a pressure of 220 GPa (2.2 million times atmospheric pressure) and began to melt when compressed above 250 GPa.
"The observation of liquid gold at 330 GPa is amazing," said Briggs. "This is the pressure at the Earth's center and is more than 300 GPa higher than previous measurements of liquid gold under high pressure."
The transition from the fcc to the bcc structure may be one of the most commonly studied phase transitions in terms of steel fabrication, where high temperatures or stresses cause a structural change between the two fcc / bcc structures. However, it is not known which phase transition mechanism is responsible for this. The results of the research team show that gold undergoes the same phase transition as a result of pressure and temperature before it melts, and future experiments on the mechanism of the transition can help to clarify important details of this important transition for the production of strong steels.  "Many of the theoretical models of gold used to understand high-pressure / high-temperature behavior did not predict the formation of a body-specific structure – just two of more than ten published papers," said Briggs. "Our findings can help theoreticians improve their models of elements under extreme compression and use these new models to study the effects of chemical bonds and assist in the development of new materials that can form in extreme conditions."
R. Briggs et al. Measurement of body-centered cubic gold and melt compression under shock compression, Physical Review Letters (2019). DOI: 10.1103 / PhysRevLett.123.045701
Study reveals new structure of gold at extremes (2019, July 31)
retrieved on July 31, 2019
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