EAGER: characterization of recovered material from SLAC Matter in Extreme Conditions quasicrystal synthesis shots

NON-TECHNICAL SUMMARY — Until 1982, scientists thought they knew the rules governing all possible ways to arrange atoms in a solid structure. Then quasicrystalline solids (or quasicrystals) were discovered. This unique group of metallic alloys have ordered atomic structures, revealed by their interactions with X-rays or electrons, but they lack the essential property of crystals, a repeating unit cell.

For another 30-years, the only known way to make quasicrystals was by careful metallurgical processing. Then two natural quasicrystalline minerals were discovered in fragments of a heavily shocked meteorite, motivating an experimental study that demonstrated synthesis of quasicrystals from crystalline starting materials in a transient high-pressure and high-temperature event.

This synthesis pathway has proven to be fruitful for discovering new compositions of quasicrystals with numerous potential applications, ranging from non-stick coatings to switchable magnets. However, recovery of quasicrystals after a shock event does not show us how and when during the event the quasicrystals nucleate and grow. This question was addressed by a novel series of recent experiments that used a laser-driven shock wave combined with a short, brilliant X-ray pulse to capture diffraction patterns in real time during quasicrystal formation.

In some of those experiments, the shocked material launched from the back of the targets was recovered. This work is dedicated to the careful, microscopic study of this recovered material. Each sample is being studied to determine what phases are present; whether they are crystalline, quasicrystalline, or amorphous; and what their compositions are.

The results are cross-referenced against the diffraction results to pin down when and how the corresponding quasicrystal diffraction patterns appeared. The result will be a basic understanding of why this method of making quasicrystals works, in the lab and in nature, and a guide to further discoveries of novel quasicrystal-forming alloys with new properties to test and explore.

TECHNICAL SUMMARY —The discovery of quasicrystal recovery from natural impacts, experimental shocks, electrical discharges, and nuclear explosions creates a new pathway to discovery and synthesis of interesting novel quasicrystals but also creates numerous basic questions. A complete understanding of the sequence of events in these experiments—and the relative importance of thermodynamic stability and of kinetic constraints—depends on combining transient diffraction data with detailed examination of the recovered products, which is the goal of this work.

The main body of the work relies on electron microscopy techniques for phase identification and characterization. Each recovered sample is isolated from the soft-catch plate, mounted, and examined optically and with secondary and back-scattered electrons. Polished surfaces are probed using electron backscatter diffraction to identify domains (as small as 1 micrometer) that yield high-contrast diffraction patterns but cannot be indexed to any unit cell.

Often, such spots show obvious five-fold symmetry axes diagnostic of the icosahedral (quasicrystal) phase. The most promising grains are characterized for composition using both energy-dispersive and wavelength-dispersive X-ray spectroscopy. Identified quasicrystal domains will be thinned to electron transparency with a focused ion beam and then studied by transmission electron microscopy to determine their defect density, six-dimensional unit cell parameters, and other characteristics.

All this information will be considered in the context of the transient X-ray results from their synthesis experiments. This work is distinctly interdisciplinary in nature and is helping to forge new pathways for discovery at the boundaries between condensed matter physics, materials science, crystallography, and geochemistry. The quasicrystal story is suitable for engaging public outreach and this work will make for excellent multimedia products describing the work and its importance in basic and applied science.

The studied and prepared specimens will be uploaded to a discoverable sample and data archive for sharing with other researchers. Participants in the work will range from undergraduate researchers to postdoctoral fellows, leveraging this research into opportunities to engage and attract the next generation of talent.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.