Polycrystalline pyridinium perchlorate has been studied over a wide range of temperatures by differential thermal analysis, proton NMR spectra and relaxation, and dielectric permittivity measurements. Two solid-solid phase transitions at 232 and 248 K have been revealed and the pyridinium cation reorientations in all three phases characterized. The higher transition appears to be the Curie point of a new ferroelectric compound.

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...

Polycrystalline pyridinium perrhenate has been studied over a wide range of temperatures by differential thermal analysis, differential scanning calorimetry, proton NMR spectra and relaxation, and dielectric permittivity measurements. Two solid-solid phase transitions at 250 and 333 K have been revealed and the pyridinium cation reorientations characterized. The higher transition appears to be the Curie point of a new ferroelectric compound.

No abstract

Articles you may be interested in[N(CH3)3H]2ZnCl4: Ferroelectric properties and characterization of phase transitions by Raman spectroscopy J. Appl. Phys. 116, 214104 (2014); 10.1063/1.4903303 Ferroelectric domain wall relaxation in Ba 0.25 Sr 0.75 Ti O 3 films displaying Curie-Weiss behavior

No abstract

The behavior of water confined in MCM-41 mesopores has been investigated by means of 1H NMR methods as a function of pore filling and temperature. The translational and rotational dynamics of water have been explored in the frame of molecular dynamics simulations providing a good understanding of the solid-state NMR results for three samples with different pore fillings: 10% (MCM-A), 45% (MCM-B), and completely filled (MCM-C). In MCM-B and MCM-C samples one can distinguish the presence of core and surface water, while all water molecules in MCM-A are attached to the surface and do not retain any translational degree of freedom. The activation energies for translational motion (MCM-B and MCM-C) obtained from NMR and MD simulations are in good agreement. For MCM-B and MCM-C samples we relate the decrease in the NMR intensity signal with decreasing temperature mainly with the loss of translational mobility of water molecules, while in the case of MCM-A the decrease is related to the freezing of rotational degrees of freedom.

International audienc