Proton nmr study of molecular dynamics and phase transitions in [NH2(CH3)2]3Sb2Cl9

Tris(dimethy1ammonium) nonachlorodiantimonate (III), [NH 2 (CH 3 ) 2 ] 3 Sb 2 Cl 9 (DMACA) crystallizes at room temperature in the monoclinic system, space group P2 1 /a. From dielectric and pyroelectric studies it was found that Dh4ACA undergoes a ferroelectric phase transition at 242 K, and below T c a polar symmetry m is expected. Dielectric dispersion studies in the microwave frequency region revealed that DMACA is a simple example of a ferroelectric crystal with an "order-disorder" mechanism for the phase transition of second order type, where polarization is due to the 180° reorientation of the dimethylammonium cations. In order to examine the dynamical properties of the dimethylammonium cation in this material we measured the spin-lattice relaxation times T 1 and T 1ρ of protons over a wide range of temperatures. The relation between NMR correlation times, obtained from an analysis of measured NMR relaxation times T 1ρ and the microscopic relaxation time obtained from dielectric dispersion studies, is discussed.