Modulated structure determination and ion transport mechanism of oxide-ion conductor CeNbO4+δ

Abstract

CeNbO4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen content (CeNbO4, CeNbO4.08, CeNbO4.25, CeNbO4.33) that shows mixed electronic and oxide ionic conduction, has been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic structures of CeNbO4.08 and CeNbO4.33. Here, we report the complex (3 + 1)D incommensurately modulated structure of CeNbO4.08, and the supercell structure of CeNbO4.33 from single nanocrystals by using a three-dimensional electron diffraction technique. Two oxide ion migration events are identified in CeNbO4.08 and CeNbO4.25 by molecular dynamics simulations, which was a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb2O9 units. However, the excess oxygen in CeNbO4.33 hardly migrates because of the high concentration and the ordered distribution of the excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO4+δ compounds elucidated here provides a direction for the performance optimization of these compounds.

Experimental and Theoretical Solid-State 29Si NMR Studies on Defect Structures in La9.33+x(SiO4)6O2+1.5x Apatite Oxide Ion Conductors

“…Understanding the mechanisms on the oxide anionic defect stabilization and oxide ion mobility is imperative in order to improve the oxide ion conductivity and design new oxide ion conductors with better performance. Recently, the materials based on tetrahedral units, for example, apatites, melilites, , and scheelites, , have attracted more and more attention on developing oxide ion conducting electrolytes owing to their high ionic conductivity. ,, The deformation and rotation flexibility of tetrahedral units facilitate the stabilization and transportation of oxygen interstitial or vacancy defects even within the low symmetric structures, ,, in contrast with the traditional fluorite and perovskite-type electrolytes. , …”

Solid oxide proton conductors beyond perovskites

“…Please do not adjust margins A variety of compositions crystallize with the scheelite, monazite or in closely related structures 162,164,165,166 and could offer an interesting crystal chemistry space for the investigation of proton conduction in systems with isolated tetrahedral moieties.…”

Oxide-ion conductivity optimization in BiVO4 scheelite by an acceptor doping strategy

“…Scheelite-type structures feature the presence of isolated tetrahedral units, which have been demonstrated to exhibit relatively high tolerance for deformation and rotation. 7,8 This peculiarity can support the stabilization and transportation of interstitial oxide ions or oxygen vacancies, which is essential for developing oxide ion conductors for use in solid oxide fuel cells.…”

“…The oxide-ion mechanism exhibited by scheelite-based materials can be seen in interstitial oxide ion conductors ( e.g. , CeNbO 4+ x , 8–10 GdNb 1− x W x O 4+ δ , 11 LaNb 1− x (W/Mo) x O 4+ δ , 12–14 and Pb 1− x La x (W/Mo)O 4+ δ 15–17 ) or vacancy-mediated oxide ion conductors ( e.g. , Ca 1− x A x WO 4− δ (A = K, Rb, Cs) 18,19 and Bi 1− x A x VO 4− δ (A = Ca, Sr)).…”

“…20–22 However, compared with other oxide ion conductors based on tetrahedral units, 23–26 there are relatively few investigations into the ionic conduction properties of scheelite compounds. In particular, oxygen-vacancy-mediated oxide ion conduction has rarely been reported for scheelite structures 8,9 or even tetrahedra-based structures. 24,25…”