Refined Structure of Metastable Ice XVII from Neutron Diffraction Measurements

TL;DR

This study precisely determined the crystal structure of metastable ice XVII using neutron diffraction, revealing its hexagonal symmetry, temperature-dependent lattice behavior, and potential for hydrogen adsorption, with stability limited to below 130 K.

Contribution

The paper provides the first detailed neutron diffraction structure of ice XVII, including accurate atomic positions and temperature-dependent lattice parameters, enhancing understanding of this metastable form.

Findings

Ice XVII has a hexagonal $P6_122$ structure with specific atomic positions.

Lattice constants vary oppositely with temperature, volume increases linearly.

Ice XVII loses metastability and transforms into ice Ih above 130 K.

Abstract

The structure of the recently identified metastable ice XVII, obtained by release of hydrogen from the C$_0$ D$_2$O-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 $P6_122$ so to obtain accurate values of the oxygen and deuterium positions. The values of the lattice constants at three temperatures between 25 to 100 K are reported, and their behavior is compared with that of ice Ih. Ice XVII is a porous solid that, if exposed to H$_2$ 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 porous ice is lost and the sample transforms into ice Ih.