Structure, phase transition and molecular motions in (C5H5NH)BiCl4

The crystal structure at 293 K of the new pyridinium compound, (C5H5NH)BiCl 4, has been determined by X-ray diffraction as monoclinic, space group Cc, Z=4. The crystal is built up of one-dimensional (BiCl4-)(n) chains and pyri dinium C5H5NH+ cations. A structural phase transition of first-order type i s detected by differential scanning calorimetry (DSC) and dilatometric meas urements at 114/110 K (on heating-cooling, respectively). Proton NMR second moment and spin-lattice relaxation time studies confirmed the order-disord er mechanism of the phase transition at 110 K. It was connected with the re orientational motion of the pyridinium cation. Dielectric investigations re veal absorption and dispersion in the audio-frequency region in both the hi gh and low temperature phases. The experimental results were analysed in th e high temperature phase in terms of the Havriliak-Negami formula. In the l ow temperature phase the Cole-Cole relation for a single relaxator was appl ied. Above the phase transition point the macroscopic relaxation time exhib its non-Arrhenius behaviour, whereas below T-c it is perfectly described by a pure Arrhenius relation.