1H nuclear magnetic resonance study of the ferroelastic and superionic phase transitions of (NH4)4LiH3(SO4)4 single crystals

We investigated the temperature dependences of the line shape, spin–lattice relaxation time, T1, and spin–spin relaxation time, T2, of the 1H nuclei in (NH4)4LiH3(SO4)4 single crystals. On the basis of the data obtained, we were able to distinguish the “ammonium” and “hydrogen-bond” protons in the crystals. For both the ammonium and hydrogen-bond protons in (NH4)4LiH3(SO4)4, the curves of T1 and T2 versus temperature changed significantly near the ferroelastic and superionic phase transitions at TC (=232 K) and TS (=405 K), respectively. In particular, near TS, the 1H signal due to the hydrogen-bond protons abruptly narrowed and the T2 value for these protons abruptly increased, indicating that these protons play an important role in this superionic phase transition. The marked increase in the T2 of the hydrogen-bond protons above TS indicates that the breaking of O–H⋯O bonds and the formation of new H-bonds with HSO4– contribute significantly to the high-temperature conductivity of (NH4)4LiH3(SO4)4 crystals.