Unveiling the Crystal Structures of Ice Polymorphs by using Neutron Diffraction under Pressure

Water exhibits remarkable polymorphism, with more than twenty crystalline ice phases identified to date. Understanding their crystal structures, hydrogen ordering, and pressure-induced structural transformations requires precise determination of hydrogen positions, making neutron diffraction an indispensable technique. However, the combination of neutron diffraction and high-pressure experiments has long been hindered by severe technical challenges arising from the small sample volumes required at high pressure. This article reviews our efforts to overcome these limitations through the development of advanced high-pressure neutron diffraction techniques, particularly the MITO system for individual pressure and temperature control and the integration of nano-polycrystalline diamond anvil cells with the PLANET beamline at J-PARC. These developments have enabled detailed investigations of hydrogen ordering and dynamics in high-pressure ice polymorphs. Studies on ice XV and the newly discovered ice XIX revealed the existence of partially ordered states and demonstrated that a single hydrogen-disordered phase can transform into distinct ordered phases depending on the pressure–temperature pathway. Time-resolved neutron diffraction measurements of the ice VII–VIII transition further uncovered a crossover between molecular rotational and proton translational motions that governs hydrogen-ordering kinetics. Extending neutron diffraction measurements beyond 100 GPa finally made it possible to directly observe the evolution of the hydrogen distribution toward the symmetrized hydrogen bond in ice X. These achievements illustrate how advances in experimental methodology have opened new opportunities for exploring the structural complexity of ice and hydrogen-bonded materials under extreme conditions.