The incompressibility and thermal expansivity of D 2 O ice II determined by powder neutron diffraction

Using high‐resolution neutron powder diffraction, the molar volume of a pure sample of D 2 O ice II has been measured, within its stability field, at 225 K, over the pressure range 0.25 < P < 0.45 GPa. Ar gas was used as the pressure medium, to avoid the formation of `stuffed ice' gas hydrates encountered when using He. The third‐order Birch–Murnaghan equation of state parameters of helium‐free D 2 O ice II, referenced to 225 K, are: V 0,225 = 306.95 ± 0.04 Å 3 (1299.7 ± 0.2 kg m −3 ), K 0,225 = 12.13 ± 0.07 GPa, with fixed at 6.0. The thermal expansivity of metastable D 2 O ice II samples recovered to ambient pressure has also been measured, over the range 4.2 < T < 160 K; above 160 K an irreversible transition to ice I c was observed. The volumetric expansion coefficient, α V , at P = 0 and T = 225 K, is predicted to be 2.48 × 10 −4  K −1 .