New sample configuration for ultrahigh pressure equation of state calibrations unveiled

In an article recently published in the Journal of Applied PhysicsAn international team of scientists from Lawrence Livermore National Laboratory (LLNL), Argonne National Laboratory, and Deutsches Elektronen-Synchrotron have developed a new sample configuration that improves the reliability of equation of state measurements in a pressure regime that was not previously achievable in the diamond anvil cell.

In terms of scale, high-quality static equation of state measurements above 5 million atmospheres, down to Neptune’s interior conditions, are achievable with this configuration.

The development of LLNL’s toroidal diamond anvil cell was revolutionary in pushing the limits of static pressure in condensed matter science. However, the next crucial step was to advance sample fabrication for more complex experiments.

Static compression experiments at pressures above 300 GPa are extremely challenging and the compression environment is often not ideal. This new sample package addresses this issue and, thanks to an improved compression environment, the quality of the equation of state data is also improved.

This work constitutes an important step towards optimized static compression experiments under these multi-megabar conditions, and provides complementary data to those from gas gun and NIF experiments conducted at LLNL.

“From this, we can report reliable calibrations of the materials’ equations of state at pressure conditions more than twice those at which most diamond anvil cell-derived equations of state have been measured,” said Claire Zurkowski, a LLNL scientist and first author of the paper.

The team used the LLNL-designed diamond toroidal anvil cell, which can consistently reach over 300 GPa with a sample chamber about 6 µm in diameter. This diameter is about 20 times smaller than the width of a human hair. In this small sample chamber, the scientists then microfabricated a set of samples in a 10-step process in which the target material is embedded in a uniform soft metal capsule, which serves as a pressure-transmitting medium.

Since samples in the diamond anvil cell are compressed by the anvils along a single axis, it is essential to redistribute this stress uniformly around the sample material to obtain a reliable measurement of the equation of state. In the case of this study, the flexible metal capsule does just that, even at the micron scale.

The experiments were conducted at Argonne National Laboratory, HPCAT Sector 16, and Deutsches Elektronen’s PETRA-III electron synchrotron. The scientists tested this methodology on molybdenum with a copper pressure-transmitting medium, but this sample set can be applied broadly.

“This work marks just the beginning of sample microfabrication in the diamond toroidal anvil cell,” Zurkowski said. “We anticipate that this sample encapsulation method will make it easy to scale static equation-of-state calibrations in materials for physics, chemistry, and planetary science into the megabar range, where static compression data are currently very limited.”

Co-authors include Rachel Lim, Olivia Pardo, Earl O’Bannon, Per Soderlind and Zsolt Jenei of LLNL and K Glazyrin of Deutsches Elektronen-Synchrotron.