Accurate and precise lattice parameters of H₂O and D₂O ice I<i>h</i>between 1.6 and 270 K from high-resolution time-of-flight neutron powder diffraction data

Accurate and precise lattice parameters for D₂O and H₂O varieties of hexagonal ice (ice Ih, space group P6₃/mmc) have been obtained in the range 1.6 to 270 K. Precision of the lattice parameters (∼0.0002% in a and 0.0004% in c for D₂O, 0.0008% in a and 0.0015% in c for H₂O) is ensured by use of the time-of-flight method on one of the longest primary neutron flight-path instruments in the world, the High-Resolution Powder Diffractometer at the ISIS neutron source. These data provide a more precise description of the negative thermal expansion of the material at low temperatures than the previous synchrotron `gold standard' [Röttger et al. (1994). Acta Cryst. B50, 644-648], including the region below 10 K where the lattice parameters saturate. The volume expansivity of both isotopologues turns negative below 59-60 K, in excellent agreement with a recent dilatometry study. The axial expansivities are highly isotropic (differing by < 1% in D<sub>2</sub>O ice Ih). Furthermore, the c/a ratio of different D<sub>2</sub>O ice samples exhibit a statistically significant dispersion of ∼0.015% below 150 K that appears to depend on the thermal history of the sample, which disappears on warming above 150 K. Similarly, H<sub>2</sub>O ice exhibits a `kink' in the c/a ratio at ∼115 K. The most plausible explanation is a freezing-in of the molecular reorientation process on cooling and subsequent relaxation on warming.