Long-range-ordered coexistence of 4-, 5-, and 6-coordinate niobium in the mixed ionic-electronic conductor γ-Ba4Nb2O9

Abstract

In a study combining high-resolution single-crystal neutron diffraction and solid-state nuclear magnetic resonance, the mixed ionic-electronic conductor Gamma-Ba4Nb2O9 is shown to have a unique structure type, incorporating niobium in 4-, 5-, and 6-coordinate environments. The 4- and 5-coordinate niobium tetrahedra and trigonal bipyrimids exist in discrete layers, within and among which their orientations vary systematically to form a complex superstructure. Through analysis and comparison of data obtained from hydrated versus dehydrated samples, a mechanism is proposed for the ready hydration of the material by atmospheric water. This mechanism, and the resulting hydrated structure, help explain the high protonic and oxide ionic conductivity of Gamma-Ba4Nb2O9. © 2013, American Chemical Society.