Orientational ordering/disordering of ions accompanied by phase transitions in pyridinium tetrafluoroborate crystal Original Research Article

The heat capacities of a pyridinium tetrafluoroborate (PyHBF4) crystal were measured in the temperature region between 6 and 372 K with an adiabatic calorimeter. The crystal showed two first-order phase transitions with heat-capacity peaks and wide tails on both the low- and high-temperature sides of the peaks. The transition temperatures were found to be 205.2 and 238.9 K for the low- and high-temperature phase transitions, respectively. The total transition entropy was calculated to be 47.2 J K−1 mol−1 as its maximum estimation, and was interpreted as indicative of the presence of 12 distinguishable orientations accessible to the pyridinium ring and four energetically lower and 12 energetically higher orientations to BF4− anion in the high-temperature disordered phase. The appearance of ferroelectric property is shortly discussed as concerned with the structural symmetry of the constituent anions in pyridinium salts. In PyHBF4, the ferroelectricity is concluded to originate from the orientational ordering of BF4− anion in which one of the four B–F bonds of each anion points to the specific, e.g. upward, direction on the 3̄-axis defined in the high-temperature disordered phase.