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

The heat capacities of a pyridinium tetrafluoroborate (PyHBF4) crystal were measured in the temperature region between 6 and 372 K with an adiabatic c alorimeter. The crystal showed two first-order phase transitions with heat- capacity peaks and wide tails on both the low- and high-temperature sides o f 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 tot al transition entropy was calculated to be 47.2 J K-1 mol(-1) as its maximu m estimation, and was interpreted as indicative of the presence of 12 disti nguishable orientations accessible to the pyridinium ring and four energeti cally lower and 12 energetically higher orientations to BF4- anion in the h igh-temperature disordered phase. The appearance of ferroelectric property is shortly discussed as concerned with the structural symmetry of the const ituent anions in pyridinium salts. In PyHBF4, the ferroelectricity is concl uded to originate from the orientational ordering of BF4- anion in which on e of the four B-F bonds of each anion points to the specific, e.g. upward, direction on the (3) over bar-axis defined in the high-temperature disorder ed phase. (C) 2000 Elsevier Science Ltd. All rights reserved.