1H and 2H NMR relaxation study on the phase transitions of (NH4)3H(SO4)2 and (ND4)3D(SO4)2 single crystals

T1, T1ρ and T2 for the 1H and 2H nuclei in (NH4)3H(SO4)2 and (ND4)3D(SO4)2 single crystals grown using the slow evaporation method were measured for phases I, II, III, IV and V. The 1H T1, T1ρ, and T2 values were found to exhibit different trends in phases II and III: T1, T1ρ and T2 for 1H do not change significantly near the phase transition at 265 K, whereas near 413 K they change discontinuously. We conclude that the NH4+ and H(SO4)2- ions do not play an important role in the III-II phase transition, but do play important roles in the II-I phase transition. The liquid-like nature of the 1H T1ρ and T2 above 413 K is indicative of the destruction and reconstruction of hydrogen bonds. Moreover, the phase transitions of the (NH4)3H(SO4)2 crystal are accompanied by changes in the molecular motion of the (NH4)+ ions. The variations with temperature of the 2H T1 and T2 of (ND4)3D(SO4)2 crystals are not similar to those observed for the 1H T1 and T2. Our comparison of the results for (NH4)3H(SO4)2 and (ND4)3D(SO4)2 crystals indicates the following: the 1H T1ρ and T2 of the (NH4)+ and H(SO4)2- ions above TC1 are characteristic of fast, liquid-like motion, which is not the case for (ND4)3D(SO4)2; and the 2H T1 of D(SO4)2- in (ND4)3D(SO4)2 is longer than the 2H T1 of (ND4)+ in contrast to the results for (NH4)3H(SO4)2 crystals