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 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.