Ferroelectric properties of pyridinium tetrafluoroborate

Abstract

The differential thermal analysis of a single crystal of pyridinium tetrafluoroborate was performed revealing two solid-solid phase transitions at 205 and 240 K. The real part of the dielectric permittivity was measured between 190 and 290 K. The large anomaly at the higher phase transition is interpreted as the Curie point of a new ferroelectric crystal. Pyridine, a strong organic base, reacts easily with many acids to form an interesting family of pyridinium compounds.In solid salts the symmetrical, planar, heteroaromatic pyridinium cation is involved in interionic hydrogen bonds. Some pyridinium salts undergo solid-solid phase transitions at low temperatures, changing the cation dynamics. The pyridinium cation, held rigidly in the crystal lattice well below the phase transition, reveals orientational disorder at room temperature. Previous NMR studies have disclosed the dynamic character of this disorder as in-plane 60' reorientational jumps of the cation about its pseudohexad axis. ' Pyridinium tetrafluoroborate C5H6NBF4 crystallizing in the trigonal space group R3rrt, with a=5.626 A, a =97. 51' and Z =1 undergoes a phase transition at 202 K. Our 'H and ' F NMR study performed over a wide range of temperature yielded a quantative description of the molecular dynamics of both ions in both solid phases. The study indicated the existence of a coupling between stochastic reorientations of cations and anions at the phase transition. This effect, probably driving the phase transition, has been discovered previously for a number of guanidinium salts. To throw more light on the problem of order-disorder phase transitions and the relevant ion mobilities in pyridinium tetrafluoroborate we have undertaken the present study of dielectric properties and differential thermal analysis of a single crystal of the cornpound.The differential thermal analysis (DTA) was performed by use of a homemade apparatus in order to confirm the existence of the phase transition and to examine its nature. Temperatures were measured using copperConstantan thermocouples with an accuracy of +1 K.The DTA results for heating and cooling at the rate of 2.5 deg/min are shown in Fig. 1. Two endothermic peaks appearing with increasing temperature at 205 and 240 K indicate clearly the occurrence of two solid-solid phase transitions. The exothermic peaks observed on cooling are only slightly shifted to lower temperatures. It is interesting to note that the heat anomaly which accompanies the higher transition is approximately twice as great as that revelant to the lower one. Distinct asymmetry of the DTA peak characterizes the higher temperature phase transition. An independent study by differential scanning calorimetry (DSC) performed for a 0,15 0.10 0.05 6 0 a -0.05 -0.10 e 0 1 5 s i~r 1 80 200 220 240 TEMPERATURE (K) 0 Cl 260 FIG. 1. DTA curves on heating and cooling (rate: 2.5 deg/min).polycrystalline sample by use of an Perkin-Elmer DSC 7 apparatus at the University of Ulm (Germany) confirms fully the results of the experiments described above. Diel...

Dielectric nonlinear properties of pyridinium tetrafluoroborate single crystal near the ferroelectric phase transition

“…1, with C p = (1.77 ± 0.02) × 10 3 K and T C = 238.3 K. The Curie-Weiss constant is similar to that presented in other papers [6,12] and the Curie temperature is one degree higher than the temperature T m , where the dielectric constant peaks. In the region of about 3 K above T m a plot of 1/ε(T) is rounded and in the range T m + 3 K we find C p = (2.61 ± 0.05) × 10 3 K and T C = 237.33 K. The temperature variations of dielectric constant measured under dc bias field of various intensities are presented in Fig.…”

“…Pyridinium tetrafluoroborate C 6 H 6 N·BF 4 (PyBF 4 ) and pyridinium perchlorate (PyClO 4 ) are particularly interesting as they exhibit successive structural phase transitions at 248 K and 232 K for PyClO 4 [6]; at 238 K and 205 K for PyBF 4 [7]. At higher temperatures both crystals undergo a paraelectricferroelectric phase transition.…”

Critical Slowing Down in Ferroelectric Pyridinium Tetrafluoroborate

“…Recently discovered ferroelectric properties of pyridinium tetrafluoroborate C 5 H 6 N BF À 4 abbreviated as PyBF 4 [1] were also detected in other pyridinium salts [2,3]. A characteristic feature of this new ferroelectric family is the occurrence of a sequence of two-phase transitions; in PyBF 4 there are the ferroelectric phase transition at T 1 238X7 K and another transition at T 2 204 K. Earlier NMR studies of pyridinium salts [4 to 7] proved the occurrence of reorientation of both pyridinium cation and BF 4 anion in the symmetric high-temperature phases.…”

Structural phase transitions in pyridinium perrhenate at high pressure

“…Depending on their symmetry and the size of anions, these compounds exhibit a large variety of interesting phenomena: phase transitions, ferroelectricity, dynamic orientational disorder of pyridinium cations, etc., [1][2][3][4]. The ferroelectric state was observed in pyridinium tetrafluoroborate PyHBF 4 (C 5 H 5 NHBF 4 ) [1], pyridinium perchlorate PyHClO 4 [2], pyridinium perrhenate PyHReO 4 [3], and pyridinium periodate PyHIO 4 [4]. Interest in studying the latter two compounds listed above is connected with the fact that their Curie temperatures are close to room temperature.…”