Abstract
Nuclear magnetic resonance measurements of the proton-lattice relaxation times T1 and T1 rho in NH4MgF3 and NH4ZnF3 between 40 and 150 K and in NH4CdF3 between 70 and 300 K are presented. The relaxation was found to be non-exponential, but fitted very well to a sum of two exponentials. Data were analysed in a three-bath model. Activation energies Ea of 760+or-50 K, 825+or-20 K and 1950+or-50 K in their low-temperature phases were found for the three compounds, respectively. The trend to an increase of Ea with increasing unit-cell size is in contradiction with the expected trend from calculations using atom-atom electrostatic potentials. The average tunnel frequencies are consistent with activation energies. Short-range forces acting between H+ and F- are assumed in order to explain these results.
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