Calorimetric Study of Glassy State. IX. Thermodynamic Properties of Stannous Chloride Dihydrate and Dideuterate Crystals

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Abstract
The heat capacity of SnCl2·2H2O and SnCl2·2D2O crystals, substances with layer structure, was measured from 13 to 300 K. Anomalies due to ordering of the hydrogen (deuterium) position were observed at TC(H2O)=217.94 K and at Tc(D2O)=234.47±0.05 K. The total entropy change of the transition was estimated to be 4.6 J K−1 mol−1 for both compounds. An exact calculation of the high temperature entropy based on the two-dimensional lattice model gave 3.13 J K−1 mol−1. The critical indices α± of the divergent heat capacity anomalies are as follows: α+(H2O)=0.53, α−H2O)=0.48, α+(D2O)=0.57, α−(D2O)=0.54. Another heat capacity anomaly was found at 140–160 K where the heat capacity values depended on the thermal history of the specimen. The enthalpy relaxation observed at the temperature region was analysed by use of the first-order rate equation, giving the activation enthalpies ΔHa(H2O)=49.9 kJ mol−1 and ΔHa(D2O)=49.8 kJ mol−1. The concept of long-range-ordered glassy state was introduced for description of the non-equilibrium state of the present substances below 140 K. A general consideration of the relation between molecular relaxation and phase change in terms of the Deborah number is presented.