Neutron-diffraction study of the phase transition in stannous chloride dihydrate

The order-disorder phase transition in two-dimensional hydrogen-bonded layers of water molecules in Sn${\mathrm{Cl}}_2$·2${\mathrm{D}}_2$O is remarkable in several respects. It has been shown that the peak in the specific heat is highly symmetric around the phase-transition temperature, and that the crystallographic symmetry of the system does not change. We present neutron-diffraction results which show that the temperature dependence of the hydrogen-site occupancies is also highly symmetric around the phase-transition temperature. These results are discussed in terms of a lattice statistical model which was proposed and solved by Salinas and Nagle. By means of a comparison with the Rhys $F$ model, this model is shown to account for the experimentally observed symmetry properties, although the model is in quantitative disagreement with the experimental data. This comparison also offers a reason for the existence of the peculiar anomaly in the dielectric constant.