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HEMI Seminar: Marius Millot (LLNL)

February 22, 2018 @ 3:30 pm - 4:30 pm

Experimental Discovery of Superionic Water with Dynamic Compression

Marius Millot, Research Scientist, High Energy Density Physics, Lawrence Livermore National Laboratory

The pressure-temperature phase diagram of water exhibits a striking degree of polymorphism with more than 15  polymorphs of molecular ice and the pressure-induced transition to the ionic ice X near 70 GPa. Upon further compression and at elevated temperature, several molecular dynamics studies have predicted that water becomes superionic, an extraordinary state with liquid-like hydrogen ions diffusing within a solid lattice of oxygen. The higher entropy of superionic ice is expected to rise its melting temperature to several thousand Kelvin and to favor the transition to new ice structures having a close-packed oxygen lattice.

We will report experimental evidence for superionic electrical conductivity above 100 GPa and 2000 K using velocimetry, pyrometry and optical property measurements of shock compressed water ice VII, as well as in-situ x-ray diffraction of ice up to 4 Mbar using reverberation compression showing that the superionic conduction indeed exists in the presence of a solid oxygen lattice as predicted three decades ago. The new XRD data also suggest the discovery of a new solid ice phase having a face-centered-cubic oxygen lattice.

In addition to providing new benchmarks for quantum theory of condensed matter, our laboratory experimental study suggests that Neptune, Uranus and many icy exoplanets may contain significant amount superionic water ice, in contrast with the prevalent picture of fluid interiors for these objects. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-724285.

Marius Millot obtained his PhD from Université Paul Sabatier in Toulouse (France) in 2009. After a postdoc at the Earth and Planetary Science Department of UC Berkeley, he is now Research Scientist at the Lawrence Livermore National Laboratory, in Livermore, CA. His research is focused on the study of matter properties at extreme pressures and temperatures for planetary science and astronomy, materials science, High Energy Density science and Inertial Confinement Fusion using dynamic compression methods and ultrafast optical diagnostics.