Structural manifestation of partial proton ordering and defect mobility in ice

Water ice is one of the most fundamental and well-studied molecular solids, which continues to provide surprising and useful insights into its emergent complexity as ever more precise experimental techniques are applied. Using one of the highest resolution neutron powder diffraction instruments in the world, I report a small but systematic distribution in the c/a axial ratios of D2O ice Ih below 160 K that depends upon both the preparation method and the thermal history of the sample. The general decrease in c/a on cooling is interpreted as a consequence of short-range partial ordering in the hydrogen-atom substructure: the synthesis-dependent variation then follows from there being a spectrum of relaxation times between samples formed by slow natural freezing, samples formed by very rapid quench-freezing and samples formed with a small amount of alkali hydroxide dopant, probably due to differences in the relative abundance of orientational versus ionic point defects.