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Physical Chemistry

Putting the squeeze to Au and Pt

Measurements at extreme pressure could help with experiments of exotic materials

by Sam Lemonick
June 3, 2021 | A version of this story appeared in Volume 99, Issue 21


Wide-angle shot of a Z machine at Sandia National Lab. A ring-shaped machine with pipes and coils and wires is bathed in blue light, with purple arcs of electricity shooting out in many directions. Yellow painted catwalks stretch above it, all under a red-lit ceiling.
Credit: Randy Montoya/Sandia National Laboratories
Scientists squeezed Au and Pt samples to 5 TPa at Sandia National Laboratories' Z machine.

Scientists have studied the behavior of gold and platinum at pressures near 1 terapascal (TPa)—a trillion pascals—several times the pressure at Earth’s center (Science 2021, DOI: 10.1126/science.abh0364). They say the new data will provide useful references for researchers studying the exotic behavior of other materials at very high pressures.

At pressures of hundreds of billions of pascals, hydrogen becomes a metal and other materials become superconductors. The extreme conditions that make these extraordinary behaviors possible also make experiments very difficult for scientists. Experiments at these pressures are often conducted within diamond anvil cells that squeeze tiny samples, and it’s not possible to directly measure the pressure experienced by the material inside. Scientists therefore rely on indirect measurements—like density—and reference materials like gold and platinum to determine the pressure inside the cell, which leaves some uncertainty in the value.

Researchers at Lawrence Livermore National Laboratory and Sandia National Laboratories set out to make more-precise measurements of the density of Au and Pt under high pressure so that scientists will be able to make better predictions about other materials. Dayne E. Fratanduono of the LLNL says his group was able to reduce uncertainties in the pressure inferences from 10–20% to just a few percent in some cases.

The researchers used two high-energy machines for their experiments. At LLNL’s National Ignition Facility, they bombarded a sample with 172 lasers to create waves of electrons that crushed their samples. Sandia’s Z machine is more like “throwing thunder bolts,” generating a current of about 20 million amperes to squeeze the samples with copper electrodes, Fratanduono says. The former produced higher pressures—up to 5 TPa—but less precise measurements, while the latter generated about 400 GPa but more-precise results. The group combined data from both experiments to draw a new phase diagram mapping density as a function of pressure for each metal.

The new data may help clarify some discrepancies seen in high-pressure experiments. For instance, several groups have claimed that they have made metallic hydrogen at varying pressures, and disagreements about the interpretation of the results have occasionally become quite bitter. Fratanduono says his group reevaluated one such study—a 2017 experiment (Science, DOI: 10.1126/science.eaal1579) that used a gold reference—and found the calculated pressures were off by a few percentage points, although his group did not publish those calculations.

In a Perspective article accompanying the study (Science 2021, DOI: 10.1126/science.abi8015), planetary geophysicist Raymond Jeanloz says the new measurements could let research groups around the world more easily compare the results of their experiments if they use these references. Fratanduono says the team hopes to repeat its work for other common reference materials like copper and tantalum.



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