The Nuclear Quadrupole Spin-Lattice Relaxation of 35Cl in Methylammonium Hexachlorostannate(IV) and Hexachloroplatinate(IV)

The 35Cl nuclear quadrupole spin-lattice relaxation time, T1 in (CH3NH3)2MCl6 (M=Sn and Pt) was measured from 4.2 to 434 K in order to clarify the dynamic aspect of the phase transitions at 156 and 125 K respectively. The relaxation process in both low-temperature phases is dominated by the rotational vibration of the octahedral [MCl6]2− ions. Far above the transition point, Ttr, the T1 decreases very rapidly with an increase in the temperature. This indicates that each octahedron reorients about its threefold axis, with the activation energies of 77±3 for [SnCl6]2− ions and 95±10 kJ mol−1 for [PtCl6]2− ions. At Ttr the log(T1⁄s) vs. kK/T curve shows a distinct minimum, as in some K2PtCl6-type compounds. This anomalous behavior can be explained in terms of a soft rotary mode of the [MCl6]2− ions. The softening is estimated to be about 60%, according to Armstrong’s approach.