Material’s Transformation Could Lead to Better-performing Devices

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A team of researchers has observed an unusual transformation in material under incredibly high pressure. Scientists captured the material, which includes manganese coupled with sulfide, changing from a soft nonconducting form to a metal and back again.

This unexpected, yet pivotal discovery means that manufacturers could see a future for this transition in the form of new components, possibly for on-off switches or conducting wires, to provide better-performing electronic devices, according to the team from the University of Nevada Las Vegas (UNLV) and the University of Rochester.

“Much of the high-pressure research we do is fundamental research,” said Dylan Durkee, a Ph.D. student at Rochester who led the experiment while he was an undergraduate at UNLV. ​“However, one could imagine next-generation memory devices that take advantage of the dramatic phase transitions in materials like this one under pressure.”

The experiments were performed at the U.S. Department of Energy’s (DOE) Argonne National Laboratory using the Advanced Photon Source (APS), a DOE Office of Science user facility. The work was done at the High-Pressure Collaborative Access Team (HP-CAT) beamline at the APS.

Durkee was responsible for most of the material sample preparation inside of a hand-held apparatus called a diamond anvil cell — essentially two diamonds that hold a sample between them and apply extreme pressure. He also conducted measurements to determine the atomic structures of the samples.

In its original form, this material does not conduct electricity. But as the pressure increases, the material changes into a conductive metal, and then back again, said Durkee, the lead author on the research team’s paper, published as an editor’s choice in Physical Review Letters.

What is extraordinary in these experiments is the physical appearance of the material while it’s in its metallic state, said Durkee.

“We typically think of metals as being shiny and reflective, which is true for almost all metals at room pressure and temperature,” Durkee said. ​“Interestingly, this one remains a blackish-reddish color while it’s metallic under pressure, which goes against our intuitive understanding of metals. Furthermore, it retains a similar color after it transforms back from a metal. These physical appearances show the nuances associated with metals and nonmetals.”

The transformation is interesting from a fundamental science viewpoint, but also from a more general perspective that materials often undergo dramatic changes under pressure, said Durkee.

“Pressure is not a parameter that we have a natural intuition for, so these types of experiments lead to surprising and exciting results,” said Durkee.

Dean Smith, an assistant physicist in Argonne’s X-ray Science Division, and Ashkan Salamat, associate professor at UNLV, performed X-ray experiments on the sample at high pressures at the APS.

“We used the APS to keep an eye on the crystal as we applied pressure,” said Smith. ​“We watched the arrangement of the atoms in the material to confirm that the changes under pressure were taking place in the same structure and were not due to any rearrangement that could happen under that pressure. Without the APS, we could not confirm that everything was happening in the same structure.”

While at the APS, Salamat said the samples were subject to pressures comparable to up to a half million times atmospheric pressures.

“Our samples were incredibly small, a tenth of the width of a human hair, and we used the incredibly bright light from the APS to be able to look at how the atoms are arranged in our materials,” said Salamat. ​“Using the diffraction capabilities of the APS, we were able to look at the way the structure of our material changed with the changes in pressure.”

“This information was important, because the results of my spectroscopy experiments suggested that the sample becomes either amorphous (without shape) or metallic under pressure,” said Durkee. ​“That the X-ray data taken from the sample shows crystallinity lends further evidence to its metallization under pressure, rather than amorphization (without a clear shape or form).”

Funding for this research was from DOE’s Basic Energy Sciences Program (BES), the National Science Foundation, and the National Nuclear Security Administration.


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