Crystal structure and phase transition of 4-aminopyridinium tetrachlorobismuthate(III), [4-NH2C5H4NH ][BiCl4 ], as studied by x-ray diffraction, dielectric, proton NMR and infrared spectroscopy

The crystal structure of 4-aminopyridinium tetrachlorobismuthate(III), [4-NH2PyH ][BiCl4 ], has been determined at 100 and 260 K by the x-ray diffraction method as monoclinic space group, P 21/c and C 2/c, respectively. Differential scanning calorimetry and dilatometry reveal one perfectly reversible discontinuous phase transition at 254/255 K (cooling/heating) with ΔS = 10.1 J mol-1 K-1. The 1H NMR spin-lattice relaxation time (T1) and second moment (M2) measurements disclose a significant change in the motional state of the cations near the phase transition temperature. Infrared spectra of polycrystalline [4-NH2PyH ][BiCl4 ] have been studied in the temperature range 10-306 K. Substantial changes in the temperature evolution of frequencies of internal modes of the 4-aminopyridinium cations near 255 K are due to the change in the dynamics of cationic moieties. The experimental results indicate that the motion of the organic cations contributes mainly to the mechanism of an 'order-disorder' transition in [4-NH2PyH ][BiCl4 ]