A research team from the Lawrence Livermore National Laboratory and the University of Edinburgh has captured information on a metastable phase of bismuth formed by nanosecond-scale shock compression using an x-ray free election laser source. Published in Applied Physics Letters (APL), these results show that materials may undergo different structural dynamics under shock compression, which could be used to identify, isolate, and explore novel states of matter.

“Bi is a material that has been extensively studied in high-pressure experiments,” says Martin Gorman, a researcher at the Lawrence Livermore National Laboratory, “because it has such a rich behavior.” Gorman is the corresponding author for the recent report in APL. High-pressure experiments on various materials, such as bismuth, were traditionally performed over a matter of seconds or minutes, using devices such as diamond anvil cells. The slow timescale of the experiment gave the bismuth lattice time to compress and relax. The corresponding structural rearrangements were observed using conventional static x-ray diffraction.

In the new experiments, the researchers dynamically compressed bismuth by initiating a fast-moving shock wave that traveled through a thin sample of the material. The pressure inside the bismuth sample, which was only 8 µm thick, rapidly increased to 5 GPa and decreased back to ambient pressure within tens of nanoseconds. The resulting structural rearrangement of the bismuth lattice were probed using the diffraction of an 80-fs, 10 keV pulse from the Linac Coherent Light Source (LCLS), the x-ray free electron laser (XFEL) at the Stanford National Accelerator Laboratory. An XFEL generates high-intensity x-ray pulses from the radiation of electrons accelerating through a specialized magnetic field.

Upon shock compression, the bismuth sample transformed from the Bi-I phase, typically found at ambient conditions, to the high-pressure Bi-V phase. Previous dynamic compression experiments had observed a similar transition as in the equilibrium case: the Bi-V relaxed to Bi-III and then to Bi-II before returning to the Bi-I phase. The most recent experiments performed using the XFEL were able to capture previously unobserved phases in the bismuth relaxation pathway.

By repeating the experiment with different time delays between the optical laser and LCLS x-ray pulse, the research team could observe the kinetics of the bismuth relaxation. Immediately after the shock wave left the sample, the bismuth remained in a mixture of the high-pressure Bi-V state, the metastable Bi-M, and Bi-II phases. After 10 ns, only the Bi-M and Bi-II phases could be observed. Closer to 20 ns, the new Bi-II´ phase appeared. “The metastable Bi-II´ state had not been observed under dynamic compression before this report,” says Kohei Ichiyanagi, a researcher at the Jichi Medical University and visiting researcher at the High Energy Research Organization in Japan.   

Researchers, like Ichiyanagi, had performed dynamic compression experiments at synchrotrons, which have less flux, time resolution, and monochromaticity than x-ray free electron lasers. Because of this, some of the bismuth relaxation dynamics were not fully resolved in the x-ray diffraction data captured at the synchrotron. “XFELs, like the LCLS, can observe previously unseen structural dynamics of materials,” Ichiyanagi says.   

The dynamics of the Bi-II´ relaxation were also reported in the APL article. The Bi-II´ phase persisted until about 40 ns, suggesting a barrier to relaxation back to the ambient Bi-I phase. After more than 50 ns, the sample relaxed to the initial Bi-I phase. “One of the promising aspects of this study is not only the observation of the Bi-II´ state on pressure release, but that this state exists at ambient pressures before slowly transitioning back,” Gorman says.

These results demonstrate that dynamic compression studies could be another way to produce novel states of matter in materials beyond bismuth. “There’s a large research effort to use dynamic pressure experiments to observe novel phases of matter,” Gorman says. Dynamic relaxation processes and cryogenic temperatures could be used to trap matter in previously unobserved states, which may have unique optical or superconducting properties. 

Read the article in Applied Physics Letters.