The roles of “ammonium” and “hydrogen-bond” protons in single crystals of the superionic conductor NH4HSeO4 by 1H NMR relaxation

The variations with temperature of the line-shape, spin-lattice relaxation time, T1, and spin-spin relaxation time, T2, of the 1H nuclei in NH4HSeO4 single crystals were investigated, and with these 1H NMR results we were able to distinguish the crystals' “ammonium” and “hydrogen-bond” protons. The line width of the signal due to the ammonium protons abruptly narrows near the temperature of the superionic phase transition, TSI, which indicates that they play an important role in this phase transition. The 1H T1 for NH4+ and HSeO4− in NH4HSeO4 do not change significantly near the ferroelectric phase transition of TC1 (=250 K) and the incommensurate phase transition of Ti (=261 K), whereas they change near the temperature of the superionic phase transition TSI (=400 K). Our results indicate that the main contribution to the low-temperature phase transition below TSI is that of the molecular motion of ammonium and hydrogen-bond protons, and the main contribution to the conductivity at high temperatures above TSI is the breaking of the O-H⋯O bonds and the formation of new H- bonds in HSeO4−. In addition, we compare these results with those for the NH4HSO4 and (NH4)3H(SO4)2 single crystals, which have similar hydrogen-bonded structure.