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

We studied the behavior of hydrogen in the mixed ionic electronic conductors gamma-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 gamma-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 gamma-III-Ba4Nb2O9 center dot 1/3H(2)O. 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 of similar to 0.37.5 H2O per formula unit. This explains the unusual local and long-range structural distortions in the hydrated form observed by neutron diffraction. Diffusion then occurs by water molecules moving between neighboring symmetry equivalent positions. These fundamentally different hydration and proton conduction mechanisms explain why 6H-Ba4Ta2O9 has the less well-defined and higher maximum water content, while gamma-Ba4Nb2O9 has the higher proton conductivity.