Observation of Plastic Ice VII by Quasi-Elastic Neutron Scattering.

Water is the third most abundant molecule in the universe and a key component in the interiors of icy moons, giant planets, and Uranus- and Neptune-like exoplanets. Owing to its distinct molecular structure and flexible hydrogen bonds that readily adapt to a wide range of pressures and temperatures, water forms numerous crystalline and amorphous phases. Most relevant for the high pressures and temperatures of planetary interiors is ice VII, and simulations have identified along its melting curve the existence of a so-called plastic phase where individual molecules occupy fixed positions as in a solid yet are able to rotate as in a liquid.

Giant Splitting of the Hydrogen Rotational Eigenenergies in the C_{2} Filled Ice.

Hydrogen hydrates exhibit a rich phase diagram influenced by both pressure and temperature, with the so-called C_{2} phase emerging prominently above 2.5 GPa. In this phase, hydrogen molecules are densely packed within a cubic icelike lattice and the interaction with the surrounding water molecules profoundly affects their quantum rotational dynamics.

Thermal behaviors during lithium diffusion in LiWO bronze studied by elastic and quasi-elastic neutron scattering.

Polycrystalline LiWO bronze has been synthesized by solid state reaction carried out in a silica tube at 10 MPa and 973 K. The sample is characterized by temperature-dependent neutron elastic and quasielastic scatterings. The room-temperature neutron powder data Rietveld refinement confirmed the space group 3̄ along with lithium occupancy found predominantly at the 6 crystallographic site.

Crossover from gas-like to liquid-like molecular diffusion in a simple supercritical fluid.

According to textbooks, no physical observable can be discerned allowing to distinguish a liquid from a gas beyond the critical point. Yet, several proposals have been put forward challenging this view and various transition boundaries between a gas-like and a liquid-like behaviour, including the so-called Widom and Frenkel lines, and percolation line, have been suggested to delineate the supercritical state space. Here we report observation of a crossover from gas-like (Gaussian) to liquid-like (Lorentzian) self-dynamic structure factor by incoherent quasi-elastic neutron scattering measurements on supercritical fluid methane as a function of pressure, along the 200 K isotherm.

Large-cage occupation and quantum dynamics of hydrogen molecules in sII clathrate hydrates.

Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples.