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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 D2O ice Ih). Furthermore, the c/a ratio of different D2O 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, H2O 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.

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