Structural changes in liquids at high pressures can be observed by shock compression

Though the mantle and outer core materials within the Earth are composed of liquids compressed at very high pressures, little is known about the structure of materials in these extreme conditions. Using high power laser pulses, Briggs et al. shock-compressed tin, an ideal element that melts at much lower pressures and temperatures than the mantle material, into its liquid phase and studied its structural changes.

The authors found that, with increasing pressure, the tin changes phase from a complex fluid to a simple fluid. The coordination number of the liquid, or the number of neighbors each of its atoms has, increases from about seven to up to 12.

“In a liquid, the changes in coordination number can be used to identify similar changes in the liquid structure and can help obtain information such as density or viscosity,” said author Richard Briggs. These properties are important for understanding the heat flow away from the inner planetary core and the nature of outer core convection, which is responsible for the Earth’s magnetic field.

Because shock compression can help the liquid reach very high pressure and temperature states that traditional techniques cannot achieve, the researchers generated shock within a tin sample using 15-nanosecond laser pulses. They conducted X-ray diffraction measurements to probe the structure of the sample and determined the coordination number of the shocked material.

Briggs noted that this approach paves the way to obtaining information about liquid structures at pressures relevant for studying planetary core compositions. “This work lays down a pathway to obtaining density of liquids, under shock compression, directly from the X-ray scattering data, a long-standing goal in our field,” he said.

Source: “Coordination changes in liquid tin under shock compression determined using in situ femtosecond X-ray diffraction,” by R. Briggs, M. G. Gorman, S. Zhang, D. McGonegle, A. L. Coleman, F. Coppari, M. A. Morales-Silva, R. F. Smith, J. K. Wicks, C. A. Bolme, A. E. Gleason, E. Cunningham, H. J. Lee, B. Nagler, M. I. McMahon, J. H. Eggert, and D. E. Fratanduono, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5127291 .