Structure, phase transitions and molecular dynamics in ferroelastic crystal pyrrolidinium hexachloroantimonate(V), [C4H8NH2][SbCl6]

The crystal structure of the pyrrolidinium hexachloroantimonate(V), [C4H8NH2][SbCl6], abbreviated PCA, has been determined by means of X-ray diffraction at 300 and 340 K. The space groups are monoclinic P21/n (phase III) and orthorhombic Pmnb (phase II), respectively. The crystal undergoes two structural phase transitions: first-order type at 356/329 K (heating/cooling) from phase (I) to (II) and second-order type at 323 K from phase (II) to (III). Dielectric studies suggest the plastic crystals behaviour above 356 K (phase I). Proton spin-lattice relaxation time (T1) and second moment (M2) of polycrystalline [C4H8NH2][SbCl6] have been determined at 77-370 K, at 90 and 25 MHz. Temperature dependence of the M2 indicates that the continuous increase in the freedom of rotational motion of the pyrrolidinium cations takes place over the phase (III). The ferroelastic domain structure of [C4H8NH2][SbCl6] is observed over the phase (III). Mechanisms of phase transitions in PCA are discussed.