The thermodynamic functions of the channel clathrate of thiourea with hexachloroethane in the temperature range 5-315 K

The heat capacity of the [(NH 2 ) 2 SC] 2 . 9 5 C 2 Cl 6 channel clathrate (thiourea-hexachloroethane) was measured adiabatically in the temperature range 5-315 K. The results were used to calculate the thermodynamic functions (entropy, the difference of enthalpies, and reduced Gibbs energy). A heat capacity anomaly was observed between 60 and 100 K with a sharp maximum at 95.60 K. An analysis of the experimental data revealed the presence of second- and first-order phase transitions that occurred simultaneously. A step change in the regular heat capacity component was observed. This change was evidence of a decrease in the mean phonon spectrum frequencies related to the first-order phase transition. The anomalous components were separated out, and their contributions to the entropy and enthalpy were determined. The entropy related to the second-order phase transition was close to Rln2. Suggestions concerning the natures of the observed phase transitions were made.