Detailed crystallographic analysis of the ice VI to ice XV hydrogen ordering phase transition

TL;DR

This study provides a detailed neutron diffraction analysis of the hydrogen ordering transition from ice VI to ice XV, revealing structural changes, the influence of pressure, and evaluating proposed structural models.

Contribution

It introduces a new computational approach for reconstructing atomistic structures of ice XV and challenges existing structural models based on diffraction data.

Findings

Hydrogen ordering causes specific lattice constant changes.

Ice XV predominantly exhibits C-type network structures.

The Pmmn structural model is incompatible with observed diffraction data.

Abstract

The D2O ice VI to ice XV hydrogen ordering phase transition at ambient pressure is investigated in detail with neutron diffraction. The lattice constants are found to be sensitive indicators for hydrogen ordering. The a and b lattice constants contract whereas a pronounced expansion in c is found upon hydrogen ordering. Overall, the hydrogen ordering transition goes along with a small increase in volume which explains why the phase transition is more difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4 GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier studies, the ice XV obtained after slow-cooling at ambient pressure is best described with P-1 space group symmetry. Using a new computational approach, we achieve the atomistic reconstruction of a supercell structure that is consistent with the average partially ordered structure derived from Rietveld refinements. This shows that C-type networks are most prevalent in ice XV but other structural motifs outside of the classifications of the fully hydrogen-ordered networks are identified as well. The recently proposed Pmmn structural model for ice XV is found to be incompatible with our diffraction data and we argue that only structural models that are capable of describing full hydrogen order should be used.