Irreversible structural changes of recovered hydrogen hydrate transforming from C0 phase to ice XVII

The present work, performed by means of ex-situ neutron diffraction measurements and combined with complementary Raman data, reports the first structural characterization of the recovered hydrogen hydrate during the transformation from the C0 phase to ice XVII. The measurement of the position of the Bragg reflections in the neutron data allows to obtain the behaviour of the lattice parameters for the C0 filled ice as a function of temperature during hydrogen release. From this analysis, we demonstrate the irreversibility of the structural changes passing from the pristine filled hydrate to the empty one, i.e. ice XVII. Meanwhile, we observe also the release of the spurious nitrogen molecules previously absorbed by the hydrate crystal when recovered at low temperature and ambient pressure. A significant release of guest molecules starts around 60 K, reaching the complete evacuation of the water structure above 100 K.