Modulated Structure Determination and Ion Transport Mechanism of Oxide-Ion Conductor CeNbO4+δ
CeNbO4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen contents (CeNbO4, CeNbO4.08, CeNbO4.25, CeNbO4.33) and showing mixed electronic and oxide ionic conduction, have been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic superstructures of CeNbO4.08 and CeNbO4.33. Here, we determinate the complex superstructures of CeNbO4.08 (89 unique atoms), CeNbO4.25 (75 unique atoms) and CeNbO4.33 (19 unique atoms) by using recently developed continuous rotation electron diffraction (cRED) technique from nano single crystals. The Ce cationic size contraction upon oxidation in CeNbO4+δ allows not only excess oxygen incorporation into the CeNbO4 host lattice at the interstitial site within the Ce cation chains (referred to as Oi), but also relaxation of the NbOn polyhedra in CeNbO4.08, CeNbO4.25, CeNbO4.33 being bridged through mixed corner/edge-sharing in 3-dimentional directions. Two kinds of oxide ion migration events are identified in CeNbO4.08 and CeNbO4.25 phases by molecular dynamic simulations, which form long-rang 3-dimensional migration pathway through the interstitial sites Oi via a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb2O9 units. However, the excess oxygen in the CeNbO4.33 phase hardly migrates because of ordered distribution of high-concentration excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO4+d compounds elucidated here provides a direction for the performance optimization of these compounds and the development of oxygen hyperstoichiometric materials for wide variety of applications.