CALORIMETRIC STUDY OF PHASE-TRANSITIONS IN PYRIDINIUM IODIDE AND PYRIDINIUM HEXAFLUOROPHOSPHATE CRYSTALS

The heat capacities of pyridinium iodide (PyHI) and pyridinium hexafluorophosphate (PyHPF6) were measured in the temperature region between 4 and 320 K with an adiabatic calorimeter. Both substances exhibited heat capacity anomalies with a sharp peak and wide skirts on both the low- and high-temperature sides of the peak. The transition temperatures were found to be 249.65 K and 217.83 K for PyHI and PyHPF6, respectively. In PyHI the entropy of transition was estimated to be in the range between 18.6 and 25.6 J K-1 mol-1, and was interpreted as indicative of the existence of 12 distinguishable orientations of the pyridinium ring in the high-temperature phase. In PyHPF, the excess entropy was estimated to be in the range between 25.0 and 36.0 J K-1 mol-1, demonstrating the cooperative progress of the reorientation of the hexafluorophosphate anion accompanied by the orientational disordering of the pyridinium ring through the transition. A characteristic picture of the orientational phase transitions in the pyridinium compounds with weak hydrogen-bond formation is discussed.