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Hydrogen ordering state of ice XXII (Fdd2)

Determine whether the orthorhombic Fdd2 phase of ice (ice XXII; Z = 304) exhibits hydrogen ordering under the reported conditions (neutron diffraction at approximately 1.48–1.74 GPa and 90–250 K), by ascertaining deuterium‑site occupancies and establishing whether the phase is fully disordered or partially/fully ordered.

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Background

Ice XXII was indexed as an F‑centered orthorhombic lattice and its oxygen sublattice refined in space group Fdd2 with Z = 304. Due to the structural complexity and data quality constraints, the authors assumed full hydrogen disorder during neutron Rietveld refinement.

They explicitly note that the available neutron data did not permit a definitive assessment of hydrogen ordering, leaving the ordering state of ice XXII unresolved.

References

We were not able to check potential hydrogen order due to structural complexity and bad signal-to-noise ratio of the neutron data (due to a reduced amount of aquaous phase compared to ice XXI data) but we suggest our Rietveld fit is good enough for a structural solution of such a complex unknown phase.

— New metastable ice phases via supercooled water (2507.14415 - Kobayashi et al., 18 Jul 2025) in Ice XXII, subsubsection “Rietveld analysis for neutron diffraction data — finilising the Fdd2 model”