Ice XVII is obtained from the so-called C0-phase (stable above 300 MPa) of the H2-H2O binary compound quenched at 77 K and room pressure, after letting the hydrogen molecules diffuse out of the crystal . It is a pure water crystal, metastable at room pressure if maintained below 130 K. Its crystal structure is intrinsically porous and presents accessible channels where hydrogen molecules were located during the synthesis process, and where other molecules (H2 or another gas) can be adsorbed again, confined in an essentially one dimensional geometry . The diameter of these channels, measured from the center of the oxygen atoms, is about 6.10 Å. This exotic and low-density water crystal adds to the list of solid structures of water possibly stable at negative pressure. By means of neutron diffraction we have recently determined the structure of ice XVII, (hexagonal, with space group P6122) . This is the second form of metastable ice experimentally obtained by removing weakly-interacting guest (the first being ice XVI, obtained from a neon clathrate ), but many low density ice structures have been theoretically studied and found to be possibly stable [4,5]. In this talk I will describe the synthesis of ice XVII, discuss its structure and properties, and describe the most recent results of a high-resolution inelastic neutron scattering experiment to study the dynamics of the H2 guest in the channels.
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