Refined Structure of Metastable Ice XVII from Neutron Diffraction Measurements

The structure of the recently identified metastable ice XVII, obtained by release of hydrogen from the C-0-structure D2O-H-2 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 P6(1)22 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 H-2 gas, may adsorb a substantial amount of it. Monitoring this effect at a constant temperature of SO 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.