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

Abstract

T1,T1ρ andT2 forthe 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, whereasnear 413 K they change discontinuously. We conclude that theNH4+ and H(SO4)2− ions do not play an important role in the III–II phase transition, but do playimportant roles in the II–I phase transition. The liquid-like nature of the1H T1ρ andT2 above413 K is indicative of the destruction and reconstruction of hydrogen bonds. Moreover, the phase transitionsof the (NH4)3H(SO4)2 crystal are accompanied by changes in the molecular motion of the(NH4)+ ions. The variations with temperature of the2H T1 and T2 of (ND4)3D(SO4)2 crystals are not similar to those observed for the1H T1 andT2. Our comparison ofthe results for (NH4)3H(SO4)2 and(ND4)3D(SO4)2 crystals indicatesthe following: the 1H T1ρ andT2 of the(NH4)+ andH(SO4)2− ionsabove TC1 are characteristic of fast, liquid-like motion, which is not the case for(ND4)3D(SO4)2; andthe 2H T1 ofD(SO4)2− in(ND4)3D(SO4)2 is longerthan the 2H T1 of(ND4)+ in contrast tothe results for (NH4)3H(SO4)2 crystals.