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Accurate and precise lattice parameters of H 2 O and D 2 O ice I h between 1.6 and 270 K from high‐resolution time‐of‐flight neutron powder diffraction data
Author(s) -
Fortes A. Dominic
Publication year - 2018
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520618002159
Subject(s) - thermal expansion , lattice (music) , powder diffractometer , atmospheric temperature range , isotropy , neutron diffraction , analytical chemistry (journal) , materials science , physics , crystal structure , crystallography , chemistry , thermodynamics , optics , diffraction , chromatography , acoustics
Accurate and precise lattice parameters for D 2 O and H 2 O varieties of hexagonal ice (ice I h , space group P 6 3 / 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 D 2 O, 0.0008% in a and 0.0015% in c for H 2 O) 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. B 50 , 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 2 O ice I h ). Furthermore, the c / a ratio of different D 2 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 2 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.