Proton NMR Study of Molecular Dynamics and Phase Transitions in Trimethyl Ammonium Hexachloro Plumbate ${[NH{(CH_3)}_3]}_2PbCl_6$ and Tetramethyl Ammonium Hexachloro Plumbate ${[N{(CH_3)}_4]}_2PbCl_6$

The proton spin lattice relaxation time $(T_1)$ measured as a function of temperature in the range 424 to 119 K in trimethyl ammonium hexachloro plumbate $[(TrMA)_2PbCl_6]$ shows phase transitions at 340 and 119 K. The observed badly resolved minima in this compound are explained on the basis of C3 reorientations of inequivalent trimethyl ammonium and methyl groups. The computed second moment values, suggest the freezing of both types of reorientation around 117 K. Proton $T_1$ studies in tetramethyl ammonium hexachloro plumbate $[(TMA)_2PbCl_6]$ in the temperature range 295 to 106 K shows a deep $T_1$ minimum at 180 K and a shallow one around 111 K, which are interpreted in terms of inequivalent tetramethyl ammonium (TMA) ions as well as $CH_3$ reorientation