Ikaite is a metastable hexahydrate of calcium carbonate, forming in aqueous conditions near freezing conditions. Neutron powder diffraction data of synthetic deuterated ikaite, collected at seven temperatures from 4 to 270 K, were refined. The linear thermal expansion coefficients are quite anisotropic, being smaller in the direction of the C—O bond. A review of the hydrogen-bonding scheme around the (CaCO03) ion pair is given and an additional weak potential hydrogen bond is suggested. Temperature-dependent growth of the atomic displacement factors of the carbonate O1 atom agrees with the previous suggestion of a possible low-frequency, hindered librational mode of the carbonate group.
