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

Abstract

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 ].