Low Temperature Spin-lattice Relaxation of Protons in Solid Ammonium Hexachloroplumbate

Nuclear spin-lattice relaxation times T 1 for protons in polycrystalline (NH 4 ) 2 PbCl 6 were measured by the NMR pulse method from room temperature to liquid helium temperature at the resonance frequencies 9, 22, and 42 MHz. A minimum in T 1 caused by the classical reorientation motion of the NH 4 + -ion was observed at about 70 K. Parameters of the reorientational motion were deduced from the T 1 data above 60 K. Additional T 1 minima were observed below 60 K. They were interpreted in terms of the temperature dependent effective tunneling splitting of the torsional ground state of the ammonium ion.