Structure and dynamics of ferroelectric pyridinium periodate

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: Cmcm→321KCmc21→210KC2. 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. (author)