Structural characterization, thermal, dielectric, vibrational properties and molecular dynamics of (C5H5NH)3BiCl6

(C5H5NH)3BiCl6 crystallizes at room temperature in the triclinic space group P  1¯. The crystal is built up of the separated BiCl63- octahedral anions and pyridinium cations. Differential scanning calorimetry (DSC) and dilatometric measurements disclose structural phase transition of first-order type at 265/302 K (cooling/heating). The dielectric investigations reveal a step-wise change of the electric permittivity at Tc characteristic of the crystals with the high-temperature plastic-like phase. Temperature-dependent infrared spectra for polycrystalline samples have been taken between 20 and 306 K to clarify the cation dynamics contribution to the mechanism of the phase transition. The modes assigned to the deformation vibration of the NH+ group in the pyridinium ring appear to be the most sensitive to the phase transition. The possible molecular motions of the C5H5NH+ cations are studied by the proton magnetic resonance (1H NMR).