Structure and dielectric properties of ferroelectric pyridinium perrhenatecrystals

The crystal and molecular structure of a new ferroelectric [C5H6N]+ReO4- wa s determined by the x-ray diffraction method at 350 K, 293 K and 220 K. In all three phases its structure is orthorhombic, with the following sequence of space groups: Cmcm double right arrow(336 K) Cmc2(1) double right arrow (250 K) Pbca. In the two high-temperature phases, the pyridinium cations ar e disordered while the anions are ordered. The low-temperature phase is ful ly ordered. The complex permittivity of a ferroelectric single crystal of pyridinium pe rrhenate was measured in the frequency range from 1 kHz to 13 MHz. The resu lts confirm the order-disorder nature of the continuous ferroelectric phase transition at T-1 = 336 K. In the vicinity of the ferroelectric phase tran sition, the phenomenon of critical slowing down was detected. The dielectri c relaxation in the paraelectric phase can be described by the Debye relaxa tion equation with a single relaxation time. The nature of the ferroelectri city is related to the degree of disorder of the pyridinium cation as well as to the appearance and the alignment of the dipole moment of the ReO4- an ion.