Phase Transition of SnCl2(H2O)x(D2O)2−x as Studied by High Resolution Heat Capacity Measurement

Abstract

Abstract The previously published heat capacity data on Sncl2(H2O)x(D2O)2−x (x=2.00, 1.97, 1.75, 1.50, 0.96, 0.50, 0.25, 0.03) were analyzed to derive anomalous contribution due to the phase transitions that occur between 218 and 235 K depending on the isotopic composition. The phase transition is of the first-order for x=2.00, 1.97, and 1.75. The entropy discontinuities at the transitions are (0.149±0.005), (0.152±0.005), and (0.133±0.010) J K−1 mol−1, respectively, out of the total transition entropies of 3.98, 3.90, and 3.89 J K−1 mol−1. The first-order transition does not occur for the other crystals for which the heat capacity anomaly is rounded. Apparent critical exponent α of the heat capacity was calculated for all of the crystals. They are in the range from 0.43 to 0.80 and have the same value below and above the transition temperature for each of the crystals in agreement with the spatial two dimensionality of the crystal structure. No evidence was found for crossing-over to the three dimensional ordering. Change of the transitional behavior with the isotopic composition was discussed in analogy with the liquid-vapor critical phenomenon. An argument based on the crystallographic symmetry of the actual crystal and the corresponding dimer lattice was advanced for this interpretation of the unusual isotope effect.