H-1 NMR study of molecular dynamics and phase transitions in (CH3NH3) 2PbBr6 and [N(CH3)(4)](2)SeCl6

The proton spin-lattice relaxation time (T-1) has been measured in methylammonium hexabromoplumbate, (CH3NH3)(2)PbBr6, in the range 77-450K and tetramethylammonium (TMA) hexachloroselenate, [N(CH3)(4) ](2)SeCl6 in the range 77-430K In (CH3NH3)(2)PbBr6, T-1 shows a slope change at 415K, a minimum at 215K, and a discontinuous jump at 167K In [N(CH3)(4)](2)SeCl6, T-1 shows a slope change at 340K and goes through a narrow symmetric minimum at 194K. Further, it exhibits a shoulder-like structure at 106K and goes through a broad minimum at 85K. Motional parameters such as activation energy and preexponential factor have been evaluated for the reorientational motion of CH3-NH3 groups and the TMA ion in different phases.