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Refined Structure of Metastable Ice XVII from Neutron Diffraction Measurements

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posted on 07.11.2016, 00:00 by Leonardo del Rosso, Francesco Grazzi, Milva Celli, Daniele Colognesi, Victoria Garcia-Sakai, Lorenzo Ulivi
The structure of the recently identified metastable ice XVII, obtained by release of hydrogen from the C0-structure D2O-H2 compound (filled ice), has been accurately measured by neutron powder diffraction. The diffraction pattern is indexed with a hexagonal cell and can be refined with space group P6122 so as to obtain accurate values of the oxygen and deuterium positions. The values of the lattice constants at three temperatures between 25 and 100 K are reported, and their behavior is compared with that of ice Ih. Ice XVII is a microporous solid that, if exposed to H2 gas, may adsorb a substantial amount of it. Monitoring this effect at a constant temperature of 50 K, we have observed that the two lattice constants show opposite behavior, a increases and c decreases, with the volume showing a linear increase. At temperatures higher than 130 K, the metastability of this form of microporous ice is lost and the sample transforms into ice Ih.