Measuring the Crystal Structure of HED Materials: X-Ray Diffraction at the Omega Laser Facility

April 2020

X-ray diffraction (XRD) is a quantitative tool for characterizing a material’s atomic structure that exploits the periodicity of atomic arrangements to produce constructive interference of x rays at specific angles scattered from parallel planes of atoms in a sample. Atomic positions within the crystal determine the x-ray peak positions and intensities. The resulting diffraction pattern is the Fourier transform of the electron density distribution and offers critical details about structures, phases, textures, and other structural parameters, such as average grain size, crystallinity, strain, and crystal defects. Therefore, XRD supplies the fingerprint of periodic atomic arrangements in a material and is a technique that has been applied to high-energy-density materials.

LLE Scientist, Danae Polsin, assembling a
powder x-ray diffraction image plate (PXRDIP)
Graduate student, Mary Kate Ginnane, working on the PXRDIP

In 2009, performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL), a method for obtaining powder x-ray diffraction data on dynamically compressed solids at multi-megabar pressures was implemented at the Omega Laser Facility. This diagnostic was used to study the structure and phase transitions of a variety of materials (low and high Z), including Ta, Sn, Al, and Mo. The diagnostic was also employed to study elements and compounds relevant to geophysics and planetary science at unprecedented high pressures. These data provided experimental constraints to the equations of states of matter at conditions that were previously only accessible using theoretical simulations. Performing experiments at the pressure and temperature conditions expected in the interiors of massive planets is vital for constraining models describing their interior structure and evolution.

High-fidelity dynamic diffraction platforms were implemented at Omega [powder x-ray diffraction image plate (PXRDIP) 2009] and the National Ignition Facility (NIF) [target diffraction in situ (TARDIS) 2016]. XRD experiments at both Omega and the NIF have been very successful in determining high-pressure crystal structures on materials with high symmetry phases. Before then, ultrahigh-pressure phase diagrams were thought to be very simple. However, our recent discoveries and calculations suggest a very different behavior. In summary,

  • these platforms have measured crystal structures up to 2 TPa (20 Mbar)
  • a wide range of thermodynamic paths can be achieved, bounded by the principal isentrope, lower temperatures, and principal Hugoniot, higher temperatures
  • solid–solid phase transitions and diffuse liquid scattering have been observed in a variety of materials
  • two independently timed x-ray exposures on the same sample have been demonstrated on the NIF

In 2019, XRD was used to enable a major scientific discovery—a new phase of water called superionic ice—at the Omega Laser Facility. Researchers from LLNL used Omega to flash freeze water into an exotic water–ice phase. Using XRD, the superionic ice’s atomic structure was able to be directly identified for the first time. The research was documented on May 8, 2019 in Nature.

LLNL Research Scientist, Federica Coppari, with an x-ray
diffraction image plate that she and her
colleagues used to discover superionic ice

“We designed the experiments to compress the water so that it would freeze into solid ice, but it was not certain that the ice crystals would actually form and grow in the few billionths of a second that we can hold the pressure-temperature conditions,” said LLNL physicist and co-lead author Marius Millot.

To document the crystallization and identify the atomic structure, the team blasted a tiny iron foil with 16 additional laser pulses to create a hot plasma, which generated a flash of x rays precisely timed to illuminate the compressed water sample once brought into the predicted stability domain of superionic ice.

“The x-ray diffraction patterns we measured are an unambiguous signature for dense ice crystals forming during the ultrafast shock-wave compression, demonstrating that nucleation of solid ice from liquid water is fast enough to be observed in the nanosecond time scale of the experiment,” said LLNL physicist Federica Coppari, co-lead author of the paper.

“In the previous work we could only measure macroscopic properties such as internal energy and temperature,” Millot added. “Therefore, we designed a new and different experiment to document the atomic structure. Finding direct evidence for the existence of crystalline lattice of oxygen brings the last missing piece to the puzzle regarding the existence of superionic water ice. This gives additional strength to the evidence for the existence of superionic ice we collected last year.”

This research gives more insight into the interior structures of giant planets in our galaxy. The scientists used the PXRDIP platform to record the data from the OMEGA experiments. PXRDIP allows the measurements of x-ray diffraction patterns of dynamically compressed materials in situ. This enabled characterization of the evolution of the atomic structure under high pressure, temperature, and strain rate. The research was performed under the auspices of the Laboratory Basic Science program at LLE.