Hydration Mechanisms and Proton Conduction in the Mixed Ionic–Electronic Conductors Ba4Nb2O9 and Ba4Ta2O9

Abstract

We studied the behavior of hydrogen in the mixed ionic–electronic conductors γ-Ba4Nb2O9 and 6H-Ba4Ta2O9 using a combination of experimental (neutron diffraction and inelastic neutron scattering) and computational (ab initio molecular dynamics) methods. Although these compounds have isostructural low-temperature polymorphs, they adopt distinct forms in the high-temperature conducting regime. We show here that they also have distinct mechanisms for hydration and ionic conduction. Hydration of γ-Ba4Nb2O9 is localized to 2-D layers in the structure that contain a 1:1 ratio of isolated but adjacent NbO4 and NbO5 polyhedra. OH– and H+ ions combine with two polyhedra, respectively, to form complete layers of NbO4OH polyhedra, giving rise to a stoichiometric hydrated form γ-III-Ba4Nb2O9·1/3H2O. Protons then diffuse through these 2-D layers by “hopping” between oxygen atoms on adjacent polyhedra. In the case of 6H-Ba4Ta2O9, hydration occurs by intercalating intact water molecules into the structure up to a maximum...