Real-time observations of the kinetics of a shock-driven phase transition using x-ray diffraction

Figure caption: The evolution of the CaF<sub>2</sub> unit cell volume versus stress/pressure obtained from dynamic (shock at DCS) and from static compression (DAC at HPCAT).
Figure caption: The evolution of the CaF2 unit cell volume versus stress/pressure obtained from dynamic (shock at DCS) and from static compression (DAC at HPCAT).

Understanding the behavior of solids under shock compression, including transformations, their pathways, and kinetics, lies at the core of contemporary static and dynamic compression science. A team led by scientists from Sandia National Laboratories is leveraging the capabilities of two sectors of the APS, HPCAT and DCS, for real-time observations of the kinetics of a shock-driven phase transition in a simple ionic solid, CaF2, a model structure in high-pressure physics.

Traditionally, shock compression research infers phase transitions from continuum-level measurements and uses corresponding static compression experiments, shock-recovery studies, or calculations to deduce their response during a shock event. The advent of synchrotron facilities where shock compression is coupled with real-time monitoring using an x-ray beam now allows for uncovering what occurs during the nanosecond time-scale when a shock wave propagates inside of a solid.

At DCS, synchrotron x-ray diffraction is coupled with plate impact launchers and photonic Doppler velocimetry is used to follow, in real time, the unfolding of a phase transition in shock-compressed CaF2. The DCS results are compared with diamond anvil cell studies at HPCAT, where x-ray diffraction under static compression and high temperatures is designed to mimic the states achieved in shock compression. The results of this work give insight into the kinetic time scale of the fluorite-to-cotunnite phase transition in under shock compression, which is relevant to a number of isomorphic compounds. A direct comparison of unit cell volumes between dynamic and static loading points to measurable structural effects of temperature on increased shock loading. Such cross-platform comparisons provide understanding of phase transitions at different time scales that improves our capability to simulate materials at extreme conditions.

This work is a multidisciplinary collaboration led by scientists from Sandia National Laboratories in collaboration with Washington State University, the University of Nevada Las Vegas and the Carnegie Institution of Washington. More in P. Kalita et al., Phys. Rev. Lett. 119, 255701, 2017.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-NA-0003525.