Deuteron NMR spectra and relaxation in fully and partly deuterated (NH4)2ZnCl4

Deuteron NMR spectra and relaxation were studied at the resonance frequency of 46 MHz in polycrystalline fully and partly deuterated (NH4)2ZnCl4 between 300 and 5 K. Spectral components confirm existence of ammonium positions with different potential symmetry, resulting in two- and threefold reorientation of ammonium ions. The temperature dependence of the spin-lattice relaxation rate discloses two time constants in the whole range. The fitting procedure allows the separation into contributions from subsystems of ions in respective potentials. Two relaxation rate maxima are attributed to ions performing threefold uniaxial reorientation at low temperatures. The lower-temperature maximum is observed at T⩽36K. With increasing temperature reorientations about remaining axes start to contribute leading to the other maximum near 100 K. The other category of ammonium ions gives rise to the maximum at about 50 K. Below this temperature the dominant motion seems to be 180∘ reorientations about one twofold axis according to observed spectra. Consistent picture of ion mobility is accomplished for 5%, 30%, 70% and 100% deuterated compounds.