Advanced Photon Source

An Office of Science National User Facility

Copper Ions Flow like Liquid through Crystalline Structures

Superionic crystals, which have become a popular topic of study only within the past five years, are a cross between a liquid and a solid. While some of their molecular components retain a rigid crystalline structure, others become liquid-like above a certain temperature, and are able to flow through the solid scaffold. In a new study, scientists probed one such superionic crystal containing copper, chromium and selenium (CuCrSe2) with high-brightness x-rays from the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS), and with neutrons from the DOE’s Spallation Neutron Source (SNS) to determine how the material's copper ions achieve their liquid-like properties. The results appeared in the journal Nature Physics.

"When CuCrSe2 is heated above 360° K, its copper ions fly around inside the layers of chromium and selenium about as fast as liquid water molecules move," said Olivier Delaire, associate professor of mechanical engineering and materials science at Duke University and senior author on the study. "And yet, it's still a solid that you could hold in your hand. We wanted to understand the molecular physics behind this phenomenon."

To probe the copper ions' behavior, Delaire and his colleagues from Duke University, Oak Ridge National Laboratory (ORNL) and Argonne National Laboratory turned to two world-class facilities: the Spallation Neutron Source at ORNL and the Advanced Photon Source at Argonne. Each machine provided a unique piece of the puzzle.

By pinging a large sample of powdered CuCrSe2 made at Oak Ridge with powerful neutrons at the SNS, the researchers got a wide-scale view of the material's atomic structure and dynamics, revealing both the vibrations of the stiff scaffold of chromium and selenium atoms as well as the random jumps of copper ions within.

For a narrower but more detailed look at vibration modes, the researchers bombarded a tiny single grain of CuCrSe2 crystal with high-resolution x-rays utilizing the HERIX spectrometer at the Argonne X-ray Science Division 30-ID-B,C beamline at the APS. This allowed them to examine how the rays scattered off its atoms and how scaffold vibrations enabled shear waves to propagate, a hallmark of solid behavior.

With both sets of information in hand, Delaire's group ran quantum simulations of the material's atomic behavior at the National Energy Research Scientific Computing Center to explain their findings. Below the phase transition temperature of 360° K, the copper atoms vibrate around isolated sites, trapped in pockets of the material's scaffold structure. But above that temperature, they are able to hop randomly between multiple available sites. This allows the copper ions to flow throughout the otherwise solid crystal.

While more work is needed to understand how the copper atoms interact with one another once both sites become occupied, the findings offer clues as to how to use similar materials in future electronic applications.

"Most commercial lithium ion batteries use a liquid electrolyte to transfer ions between the positive and negative terminals of the battery," Delaire said. "While efficient, this liquid can be dangerously flammable, as many laptop and smartphone owners have unfortunately discovered."

"There are variants of superionic crystals that contain ions like lithium or sodium that behave like the copper in CuCrSe2," Delaire said. "If we can understand how superionic crystals work through this study and future research, we could perhaps find a better, solid solution for transporting ions in rechargeable batteries."

See: Jennifer L. Niedziela 1,2‡*, Dipanshu Bansal2**, Andrew F. May1, Jingxuan Ding2, Tyson Lanigan-Atkins2, Georg Ehlers1, Douglas L. Abernathy1, Ayman Said3, and Olivier Delaire2***, “Selective breakdown of phonon quasiparticles across superionic transition in CuCrSe2,” Nat. Phys., published on line 08 October 2018. DOI: 10.1038/s41567-018-0298-2

Author affiliations: 1Oak Ridge National Laboratory, 2Duke University, 3Argonne National Laboratory

Present address: Oak Ridge National Laboratory

Correspondence: *, **, ***

We are grateful to J. Z. Tischler for algorithms enabling deconvolution of the energy resolution from the inelastic x-ray phonon scattering data. J.L.N., J.D., and T.L.-A. were supported as part of the S3TEC EFRC, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE) Office of Science-Basic Energy Sciences under award no. DE-SC0001299. D.B. and O.D. were supported by the U.S. DOE  Office of Science-Basic Energy Sciences, Materials Sciences and Engineering Division, under the Early Career Award no. DE-SC0016166 (principal investigator O.D.). A.F.M. was supported by the U.S. DOE  Office of Science-Basic Energy Sciences, Materials Sciences and Engineering Division. The research at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE. Ab initio molecular dynamics calculations were performed using resources of the National Energy Research Scientific Computing Center, a U.S. DOE Office of Science User Facility supported by the Office of Science of the U.S. DOE under contract no. DE-AC02-05CH11231. Density functional theory simulations for this research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. DOE under contract no. DE-AC05-00OR22725. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of  Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.

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