Structure and dielectric properties of ferroelectric pyridinium perrhenate crystals

The crystal and molecular structure of a new ferroelectric [C5H6N]+ReO4- was 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 are disordered while the anions are ordered. The low-temperature phase is fully ordered. The complex permittivity of a ferroelectric single crystal of pyridinium perrhenate was measured in the frequency range from 1 kHz to 13 MHz. The results confirm the order-disorder nature of the continuous ferroelectric phase transition at T-1 = 336 K. In the vicinity of the ferroelectric phase transition, the phenomenon of critical slowing down was detected. The dielectric relaxation in the paraelectric phase can be described by the Debye relaxation equation with a single relaxation time. The nature of the ferroelectricity 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- anion.