Mechanism of protonic conductivity in an NH4HSeO4 crystal

The chemical exchange of deuterons in a partly deuterated ammonium hydrogen selenate crystal is investigated by deuteron magnetic resonance (²H NMR) spectroscopy over a wide range of temperatures. The changes observed in the line shape of the NMR spectra at temperatures above 350 K are characteristic of chemical exchange processes. The exchange processes are thoroughly examined by two-dimensional ²H NMR spectroscopy. It is established that, over the entire temperature range, only deuterons of hydrogen bonds are involved in the exchange and the rates of exchange between deuterons of all types are nearly identical. No deuteron exchange between the ND₄ groups and hydrogen bonds is found. A new model of proton transport in ammonium hydrogen selenate is proposed on the basis of the experimental data. This model makes it possible, within a unified context, to explain all the available experimental data, including macroscopic measurements of the electrical conductivity.