The roles of “ammonium” and “hydrogen-bond” protons in single crystals of the superionic conductor NH 4HSeO 4 by 1H NMR relaxation

Abstract

The variations with temperature of the line-shape, spin–lattice relaxation time, T 1, and spin–spin relaxation time, T 2, of the 1H nuclei in NH 4HSeO 4 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, T SI, which indicates that they play an important role in this phase transition. The 1H T 1 for NH 4 + and HSeO 4 − in NH 4HSeO 4 do not change significantly near the ferroelectric phase transition of T C1 (=250 K) and the incommensurate phase transition of T i (=261 K), whereas they change near the temperature of the superionic phase transition T SI (=400 K). Our results indicate that the main contribution to the low-temperature phase transition below T SI is that of the molecular motion of ammonium and hydrogen-bond protons, and the main contribution to the conductivity at high temperatures above T SI is the breaking of the O–H⋯O bonds and the formation of new H– bonds in HSeO 4 −. In addition, we compare these results with those for the NH 4HSO 4 and (NH 4) 3H(SO 4) 2 single crystals, which have similar hydrogen-bonded structure.