Slightly Hydrogen-Ordered State of Ice IV Evidenced by In Situ Neutron Diffraction

Ice IV is a metastable high-pressure phase of ice in which the water molecules exhibit orientational disorder. Although orientational ordering is commonly observed for other ice phases, it has not been reported for ice IV. We conducted in situ powder neutron diffraction experiments for DCl-doped D₂O ice IV to investigate its hydrogen ordering. We found abrupt changes in the temperature derivative of unit-cell volume, dV/dT, at ∼120 K, and revealed a slightly ordered structure at low temperatures based on the Rietveld method. The occupancy of the D1 site deviates from 0.5 in particular; it increased when samples were cooled at higher pressures and reached 0.174(14) at 2.38 GPa, 58 K. Our results evidence the presence of a low-symmetry hydrogen-ordered state corresponding to ice IV. It seems, however, difficult to experimentally access the completely ordered phase corresponding to ice IV by slow cooling at high pressure.