Unveiled by Bridgman's pioneering discovery of five solid phases in 1912, the extraordinary polymorphism of H2O continues to stimulate numerous studies that have documented at least seventeen crystalline and several amorphous ice structures. This unique behavior is due in part to the geometrical frustration of the weak intermolecular hydrogen bonds, as well as the sizable quantum motion of the lightweight hydrogen ions. Among many surprising properties including metastability and kinetic effects , H2O has been predicted to become superionic when submitted to extreme pressures exceeding 1 million atmospheres (100 GPa) and temperature above 2,000 K. Numerical simulations of superionic water ice suggest that the characteristic diffusion of charged hydrogen ions through the empty sites of the oxygen solid lattice should not only enable a surprisingly high ionic conductivity, it should also dramatically increase its melting temperature to several thousand Kelvin and favor new ice structures having a close-packed oxygen lattice. Because confining hot, dense H2O in the laboratory is extremely challenging, experimental data are scarce.
Recent optical experiments along the locus of shock states (Hugoniot) for water ice VII evidenced superionic conduction and thermodynamic signatures for melting[1] but did not determine the microscopic structure of superionic ice.
We will report new experiments [2] using laser-driven shock-waves to simultaneously compress and heat liquid water samples to 100-400 GPa and 2,000-3,000 K. In-situ x-ray diffraction (XRD) measurements show that water solidifies into nanometer-sized ice grains within a few nanoseconds and provide unambiguous evidence for a crystalline lattice of oxygen in superionic water ice. Further, the x-ray diffraction data allow us to document ice's compressibility at unprecedented conditions and unravel a temperature- and pressure-induced phase transformation from a body-centered-cubic ice (likely ice X) to a new face-centered-cubic, superionic ice that we name ice XVIII.
[1] Millot, M. et al. Experimental evidence for superionic water ice using shock compression. Nat. Phys. 14, 297–302 (2018).
[2] Millot, M. et al. Nanosecond X-ray diffraction of shock-compressed superionic water ice. Nature 569, 251–255 (2019).
Prepared by LLNL under Contract DE-AC52-07NA27344.