Structure, phase transitions and molecular dynamics in 4-aminopyridinium hexachloroantimonate(V), [4-NH2C5H4NH][SbCl6]

4-Aminopyridinium hexachloroantimonate(V), [4-NH2C5H4NH][SbCl6], abbreviated to 4APCA, crystallizes at room temperature in the monoclinic system, space group P21/c (phase III). The crystal undergoes two reversible phase transitions of first order type at 386/346 K (II↔III) and at 467/466 K (I↔II) on heating–cooling runs. Dielectric studies suggest the plastic crystalʹs behavior above 386 K (phase II). The proton NMR second moment (M2) and the1H NMR spin-lattice relaxation time (T1) for 4APCA measured between 77 and 420 K revealed the C2 reorientation of the 4-aminopyridinium cations. The mechanism of the phase transitions in 4APCA at 386 K is discussed.