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.

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Abstract. Measurements of spin-lattice relaxation time T 1 for resorcinol have been made by the proton NMR technique using the saturation method in the temperature range 280 -380 K and pressure up to 800 MPa. The pressure-induced -β transition evolved through two phase coexistence range was observed. The crystal structure and vibrational spectra of the resorcinol have also been studied by means of X-ray diffraction and Raman spectroscopy at pressures up to 19 GPa in temperature range 290 -380 К. In experiments with a rapid pressurization rate, suppression of α-β polymorphic transition in resorcinol occurs. In experiments with a slow pressurization rate, two structural phase transitions, between orthorhombic α and β phases at P = 0.4 GPa and from β to another orthorhombic  phase at P=5.6 GPa, were observed. At pressures above 12.5 GPa, a gradual transformation to the amorphous phase was revealed. The lattice parameters, unit cell volumes and vibration modes as functions of pressure and temperature were obtained for the different polymorphic modifications of resorcinol.

The weak-field dielectric dispersion ͑100 Hz-1.8 GHz͒ studies both of pure and Fe 2ϩ /Fe 3 modified Cd 2 Nb 2 O 7 ceramics over the temperature range of 90-380 K are presented and discussed from the viewpoint of relaxor and glassy properties of the system. It is revealed that In the latter, at T C ϭ196 K the dielectric relaxation due to the motion of ferroelectric domain walls driven by an external ac electric field is observed. A polydispersive dielectric response of Cd 2 Nb 2 O 7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior ͑like PMN͒. Near the characteristic freezing temperature of the zero-field-cooled state (T f ϭ183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below T f the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd 2 Nb 2 O 7 dominating far below T C ͑around 150 K͒ and that of Fe 2ϩ /Fe 3ϩ doped CdNb 2 O 6 between 90 and 380 K are typical of glass-forming systems at temperature far above T glass . The relaxational process is characterized by ͑i͒ a significant frequency dependence of the peak permittivity position, ͑ii͒ non-Arrhenius behavior, and ͑iii͒ increasing asymmetry of the dielectric absorption spectrum at the low-frequency side with decreasing temperature, without broadening the relaxation-time distribution and freezing the peak-absorption frequency. It is proposed that although the nature of structural disorder in Cd 2 Nb 2 O 7 pyrochlore and Fe 2ϩ /Fe 3ϩ doped CdNb 2 O 6 columbite is different, in both systems the off-center displacements of the A-site ions act as a random field and are responsible for the relaxor and dipolar glass-like behavior upon cooling. The Debye-like HF dielectric relaxation ͑1 MHz-1.8 GHz͒ observed both in Cd 2 Nb 2 O 7 and its isostructural analog Cd 2 Ta 2 O 7 at RT and higher ͑a centrosymmetric phase͒ is attributed to fluctuations in polarization of the dynamically reoriented O͑7th͒-Cd-O͑7th͒ dipoles due to dynamical off-center location of Cd ions.

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