The crystal and molecular structure of a new ferroelectric from the
pyridinium salt group [C5H5NH]+IO4- was
determined by the x-ray diffraction method at 350, 300 and 100 K. The high
temperature and intermediate phases are orthorhombic, while the low
temperature phase is monoclinic, with the following sequences of space
groups and continuous phase transitions: . The two orthorhombic phases are isostructural with PyReO4, where the
pyridinium cation is disordered and the periodate anion is ordered. The low
temperature phase is well ordered. The intermediate and low temperature
phases are ferroelectric. Measurements of spin-lattice relaxation time and
complex permittivity have been performed for polycrystalline and
monocrystalline samples as a function of pressure and temperature. The
potential shape and heights of the energy barriers for cation reorientations in
ferroelectric phases have been proposed. The complex permittivity
measurements indicate the order-disorder character of the ferroelectric
phase transition and continuous slowing of the cation dynamics with
decreasing temperature.
Measurements of spin-lattice relaxation time T 1 , second moment M 2 and neutron scattering have been performed for a polycrystalline sample of pyridinium nitrate as a function of hydrostatic pressure and temperature. The structure of this compound has also been analysed by the HF/6-31 G method. The results of the measurements and calculations have confirmed that reorientation of the pyridinium cation takes place between the potential minima of different values, and the asymmetry parameter decreases with increasing temperature. The temperature dependence of the asymmetry parameter is modified by the pressure applied. The presence of the hydrogen bond in this compound implies a very small activation volume and is responsible that the crystal does not undergo a phase transition to the disordered phase.
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