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 P2(1)/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 (T-1) and second moment (M-2) of polycrystalline [C4H8NH2][SbCl6] have been determined at 77-370 K, at 90 and 25 MHz. Temperature dependence of the M-2 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. (c) 2005 Elsevier SAS. All rights reserved.