Polarisation and Energy Barriers in Ferroelectric Pyridinium Perchlorate

Abstract

Polycrystalline sample of pyridinium perchlorate was studied by NMR method in the temperature range from 133 to 313 K. Assuming a reorientation of pyridinium cations over inequivalent potential energy barriers in the low-temperature and intermediate phases a temperature dependence of an asymmetry parameter was determined from analysis of 2 H line shape. By using the known potential shape and population of its minima, a temperature dependence of the spontaneous polarisation was calculated. The polarisation determined in this way compared with that measured by the pyroelectric effect revealed the mixed type of ferroelectricity in the crystal studied: the order-disorder and displacive one.

Ferroelectricity of Inclusion Compounds of Thiourea with Pyridinium Iodide and Nitrate

“…The spontaneous polarizations of the two compounds have been estimated as about 1 μC/cm 2 . The spontaneous polarization measured in pyridinium salts varied from about 1 to 3 μC/cm 2 , ,,, whereas in a very well-known ferroelectric, triglycine sulfate, TGS, crystal, the spontaneous polarization reaches about 3 μC/cm 2 …”

“…In the crystals of displacement type, spontaneous polarization appears as a result of relative displacement of ions or molecules, while in the crystals of order−disorder type, it appears as a result of ordering of electric dipoles of ions or molecules. , Spontaneous polarization can be determined experimentally from the pyroelectric effect or hysteresis loop. As yet, there have been only a few attempts at calculation of the spontaneous polarization, − as its calculation is difficult but seems to be informative about the nature of this phenomenon.…”

“…To explain the nature of ferroelectricity in these salts, spontaneous polarization was calculated for the crystals (PyH)BF 4 , (PyH)ClO 4 , and (PyH)IO 4 . − , NMR ( 1 H and 2 H) results have shown that in all pyridinium salts the cation reorientations take place around the axis perpendicular to its plane, in the intermediate and low-temperature phases, between the potential energy minima of different values. , So, on the basis of the cation dynamics, a model of 6-fold potential barrier, of different barrier heights, was assumed for (PyH)BF 4 and (PyH)ClO 4 , while for (PyH)IO 4 , the shape of the potential for the cation reorientations was established on the basis of the populations of the positions in the pyridinium cation by nitrogen atoms determined by neutron diffraction. − The temperature dependence of the 2 H NMR line shape permitted determination of the populations of the particular potential energy minima and hence also the polarization as a function of temperature. A very good agreement between the measured and calculated values of the spontaneous polarization for (PyH)BF 4 , (PyH)ClO 4 (in high- and low-temperature phases), and (PyH)IO 4 (all phases) permits classification of these pyridinium salts to the order−disorder type …”

Dynamics in molecular and molecular-ionic crystals: A combined experimental and molecular simulation study of reorientational motions in benzene, pyridinium iodide, and pyridinium nitrate

“…Much effort has been made to prove that the pyridinium cation really reorients in a temperature dependent asymmetric potential, as such potential allows describing the ferroelectric properties of these compounds. Thus, the compounds pyridinium iodide, 2, 14-16 pyridinium tetrafluoroborate, 17 pyridinium perchlorate, 18,19 pyridinium periodate, [20][21][22] pyridinium perherate, 23,24 and pyridinium nitrate 4 have been studied by 1 H nuclear magnetic resonance (NMR) and inelastic neutron scattering as a function of temperature and pressure and by 2 H NMR and quasielastic neutron scattering (QENS) as a function of temperature. A detailed analysis of the dynamics of molecules or ions provides important information to understand their properties or the mechanism of their phase transitions.…”

Temperature Dependence of Spontaneous Polarization in Order−Disorder Pyridinium Periodate Extracted from 2H NMR Data

“…As yet, only a few attempts at calculations of spontaneous polarization have been made. [5][6][7][8][9][10][11][12] The problem is not easy, but the calculation of spontaneous polarization seems to be the best way to understand the nature of ferroelectricity in the compounds studied.…”