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

Abstract

The crystal structure at 293 K of the new pyridinium compound, (C₅H₅NH)BiCl₄, 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 pyridinium C₅H₅NH⁺ cations. A structural phase transition of first-order type is detected by differential scanning calorimetry (DSC) and dilatometric measurements at 114/110 K (on heating–cooling, respectively). Proton NMR second moment and spin–lattice relaxation time studies confirmed the order–disorder mechanism of the phase transition at 110 K. It was connected with the reorientational motion of the pyridinium cation. Dielectric investigations reveal absorption and dispersion in the audio-frequency region in both the high and low temperature phases. The experimental results were analysed in the high temperature phase in terms of the Havriliak–Negami formula. In the low temperature phase the Cole–Cole relation for a single relaxator was applied. Above the phase transition point the macroscopic relaxation time exhibits non-Arrhenius behaviour, whereas below T_c it is perfectly described by a pure Arrhenius relation.