Proton spin-lattice relaxation and ammonium tunnelling in (NH₄)₂ZnCl₄

Abstract

Effects of rotational tunnelling on the proton spin-lattice relaxation were studied in (NH4)2ZnCl4 as functions of both temperature and the resonance frequency. Several tunnelling-related T1 minima were found. At 20K the tunnel frequencies were deduced to be 13 MHz, 24.5 MHz, 37 MHz and 51.5 MHz corresponding to the separation between the nondegenerate and the doubly degenerated T levels, to the T-A splittings and to the E-A splitting, respectively. The smallest tunnel frequency (13 MHz) was observed to branch into two above 20K. The higher frequency branch could not be detected above 26K because of a broadening of the corresponding T1 minimum. The broadening suggests a presence of two motions which was supported by experiments on the spin-lattice relaxation of the dipolar energy.