Recent studies have shown that ammonium langbeinites have low-temperature phase transitions uncharacteristic of the rest of the langbeinite family. In this study, proton NMR spin-lattice relaxation times ${\mathit{T}}_1$ and spin-spin relaxation times ${\mathit{T}}_2$ for powder samples of (${\mathrm{NH}}_4$${)}_2$${\mathrm{Mg}}_2$(${\mathrm{SO}}_4$${)}_3$ are reported in the temperature range from 77 to 373 K. The ammonium ions occupy two different kinds of sites in the crystal. The measured barrier to thermally activated ammonium reorientations is found to be 2.0 and 3.3 kcal/mol for the two sites, respectively. Additionally, there is no observable discontinuity in ${\mathit{T}}_1$ at the reported phase transition at 161 K, in agreement with heat-capacity studies, suggesting that the transition mechanism may be different from those of most langbeinites.

• Received 20 June 1994

DOI:https://doi.org/10.1103/PhysRevB.51.11315