Effect of cationic substitution on the double-well hydrogen-bond potential in [K1−x (NH4) x ]3H(SO4)2 proton conductors: a single-crystal neutron diffraction study

Abstract

The structure of the mixed crystal [K1-x(NH4)x]3H(SO4)2 as obtained from single-crystal neutron diffraction is compared with the previously reported room-temperature neutron structure of crystalline K3H(SO4)2. The two structures are very similar, as indicated by the high value of their isostructurality index (94.8%). It was found that the replacement of even a small amount (3%) of K+ with NH4+ has a significant influence on the short strong hydrogen bond connecting the two SO42- ions. Earlier optical measurements had revealed that the kinetics of the superionic transition in the solid solution [K1-x(NH4)x]3H(SO4)2 are much faster than in K3H(SO4)2; this reported difference in the kinetics of the superionic phase transition in this class of crystal is explained on the basis of the difference in strength of the hydrogen-bond interactions in the two structures.