2020, Schuster, A.K., Hartley, N.J., Vorberger, J., Döppner, T., Van Driel, T., Falcone, R.W., Fletcher, L.B., Frydrych, S., Galtier, E., Gamboa, E.J., Gericke, D.O., Glenzer, S.H., Granados, E., MacDonald, M.J., MacKinnon, A.J., McBride, E.E., Nam, I., Neumayer, P., Pak, A., Prencipe, I., Voigt, K., Saunders, A.M., Sun, P., Kraus, D.

We present measurements of the nucleation rate into a diamond lattice in dynamically compressed polystyrene obtained in a pump-probe experiment using a high-energy laser system and in situ femtosecond x-ray diffraction. Different temperature-pressure conditions that occur in planetary interiors were probed. For a single shock reaching 70 GPa and 3000 K no diamond formation was observed, while with a double shock driving polystyrene to pressures around 150 GPa and temperatures around 5000 K nucleation rates between 1029 and 1034m-3 s-1 were recorded. These nucleation rates do not agree with predictions of the state-of-the-art theoretical models for carbon-hydrogen mixtures by many orders of magnitude. Our data suggest that there is significant diamond formation to be expected inside icy giant planets like Neptune and Uranus. © 2020 authors. Published by the American Physical Society.

2020, Hernandez, J.-A., Morard, G., Guarguaglini, M., Alonso-Mori, R., Benuzzi-Mounaix, A., Bolis, R., Fiquet, G., Galtier, E., Gleason, A.E., Glenzer, S., Guyot, F., Ko, B., Lee, H.J., Mao, W.L., Nagler, B., Ozaki, N., Schuster, A.K., Shim, S.H., Vinci, T., Ravasio, A.

We report in situ structural measurements of shock-compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X-ray diffraction to laser-driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X-ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post-perovskite have been indirectly reported in microsecond-scale plate-impact experiments. Therefore, we interpret the high-pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post-perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO3 on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments. © 2020. The Authors.