Structure, phase transitions and molecular dynamics in 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4] Original Research Article

Crystal structure of the 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4], has been determined at 240 K by X-ray diffraction as monoclinic, space group, P21/n, Z=8. Differential scanning calorimetry and dilatometric studies indicate the presence of two reversible phase transitions of first order type, at 335/339 and 233/289 K (cooling/heating) with ΔS=0.68 and 2.2 J mol−1 K−1, respectively. Crystal dynamics is discussed on the basis of the temperature dependence of the 1H NMR spin–lattice relaxation time T1 and infrared spectroscopic studies. The low temperature phase transition at 233 K of an order–disorder type is interpreted in terms of a change in the motional state of the 4-methylpyridinium cations. The phase transition at 335 K, probably of a displacive type, is characterised by a complex mechanism involving the dynamics of both the cationic and anionic sublattice. The 1H NMR studies show that the low temperature phase III is characterised only by the dynamics of the CH3 groups.