Is gold always stable?
THE UNIQUENESS of gold and its appreciation as a valuable throughout history is closely related to its exceptional stability to chemical reactions and extreme pressures and temperatures. Gold was considered as a synonym of immovability and constancy (remember the wedding rings!). But gold is not as noble and stable as previously thought.
Material structure
This is the conclusion of an international team of researchers from Germany, France and Sweden who came to the European Synchrotron radiation Facility (ESRF) to study the structure of this material at high pressure.
They present their results in Physical Review Letters. In order to carry out their experiments, scientists used two beamlines of the ESRF combined with a new instrument at the Bayerisches Geoinstitut.
Electrically heated
The sample was placed inside a diamond anvil cell, which was then electrically heated externally, according to an ESRF press release.
This allowed them to study gold at the pressures of the Earth's core, that is, at a depth of 5500 km from the surface.
Advances in high-pressure techniques require standards, which are applicable at a multimegabar pressure range.
The experiments have shown that at pressures above around 240 GPa gold adopts a hexagonal-close packed structure.
Large pressure and temperature stability of the cubic gold phase and its high isothermal compressibility make gold an ideal material as a pressure marker at high pressure-high temperature experiments at pressures above 100 GPa.
Natural limit
The pressure-induced phase transition found in gold above 240 GPa places a `natural' limit on the application of cubic gold as a standard. These results confirm the theoretical predictions about the phase changes in gold. — Our Bureau
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