1977 Volume 50 Issue 12 Pages 3167-3176
The neutron-diffraction results for deuterated single crystals have shown definitely that the phase transition is ascribable to the ordering of the hydrogen atoms. The ordered arrangement below the transition temperature (Tt=234 K) agrees well with the NMR results. No doubling of the unit cell or loss of the center of symmetry can be detected. In the disordered phase above Tt, four deuterons of two non-equivalent water molecules are distributed into seven sites. The occupancy factors of the individual sites have been refined by least-squares methods under different constraints. Bernal and Fowler’s ice rule is obeyed in the H-bonded water layers, and the water molecules of crystallization scarcely dissociate at all. The crystal structure has been redetermined by the X-ray diffraction method at 209 and 223 K, using an SnCl2·2H2O single crystal. No distinct change in the structure of non-hydrogen atoms has been disclosed on passing through the transition point (Tt=218 K). However, in the close vicinity of Tt, the a and c lattice parameters and the three H-bonded O···O distances steeply change in association with the ordering of the hydrogen atoms.
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