Laser-driven compression experiments to unravel complex material behavior at extremes: the discovery of superionic water ice

Dr. Federica Coppari, Lawrence Livermore National Laboratory

Goergen 101

Abstract:

The use of lasers to induce extreme compression states has enabled the study of material properties and equations of state at unprecedented pressures and temperature conditions. By carefully designing the laser pulse shape (i.e. laser power vs time), one can tune the compression history of the sample and reach a specific pressure-temperature state. In this way, lasers can be used to recreate in the laboratory the conditions existing in planetary interiors. The combination of laser-driven compression and x-ray diagnostics allow us to probe these extreme pressure-temperature states in-situ, providing a unique picture of the transformations taking place in high-energy-density matter. Structural probes, such as X-ray diffraction (XRD) have been developed at the Omega laser (University of Rochester, NY) to investigate phase transitions occurring on nanosecond time scales as a result of laser-driven dynamic compression. This experimental platform has been used to demonstrate the nanosecond solidification of liquid water into a superionic ice form, Ice XVIII. Since Bridgman’s discovery of five solid water ice phases in 1912, studies on the polymorphism of H₂O have documented seventeen crystalline and several amorphous structures, as well as metastability and kinetic effects. Particularly intriguing is the prediction that water becomes superionic —with liquid-like hydrogens diffusing through the solid lattice of oxygen— when subjected to extreme pressures exceeding 100 GPa and temperatures above 2000 K. Because confining such hot and dense H₂O in the laboratory is extremely challenging, experimental data are scarce. Optical measurements along the Hugoniot curve of ice VII showed evidence of superionic conduction at about 150 GPa and 3000 K, but did not confirm the microscopic structure of superionic ice. In situ X-ray diffraction was used to show that under the same conditions, water solidifies within a few nanoseconds into nanometer-sized ice grains that exhibit unambiguous evidence for the crystalline oxygen lattice of superionic water ice. In addition to representing a critical test to numerical method for high pressure/high temperature condensed matter, the experimental discovery of superionic water ice at conditions expected deep inside ice giant planets provides new constraints to planetary models describing the interior structure or Uranus and Neptune.

 Bio:

Dr. Coppari received a B.Sc. and M.Sc. in Physics from University of Camerino (Italy) in 2004 and 2007 respectively and a Ph.D. in Physics from the University Pierre et Marie Curie, Paris (France) in 2010. Her PhD thesis work focused on understanding pressure-induced amorphous-amorphous phase transitions using diamond anvil cells and synchrotron-based X-ray diffraction and EXAFS experiments. In 2011 she joined the shock physics group at Lawrence Livermore National Laboratory as a postdoctoral research scientist, working on the study of material behavior at extreme conditions by laser-driven compression and diagnostic development for HED material science experiments, in particular x-ray diffraction and Extended X-ray Absorption Fine Structure to study phase transitions and equations of state. Her research interests and expertise span from high pressure/temperature condensed matter and disordered systems to high energy density and planetary science.

Location: Goergen 101

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