Long-Range-Ordered Coexistence of 4‑, 5‑, and 6‑Coordinate Niobium in the Mixed Ionic-Electronic Conductor γ‑Ba₄Nb₂O₉

In a study combining high-resolution single-crystal neutron diffraction and solid-state nuclear magnetic resonance, the mixed ionic-electronic conductor γ-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 γ-Ba₄Nb₂O₉.