Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure

Abstract

Tetrahedral units can transport oxide anions via interstitial or vacancy defects owing to their great deformation and rotation flexibility. Compared with interstitial defects, vacancy-mediated oxide-ion conduction in tetrahedra-based structures is more difficult and occurs rarely. The isolated tetrahedral anion Scheelite structure has showed the advantage of conducting oxygen interstitials but oxygen vacancies can hardly be introduced into Scheelite to promote the oxide ion migration. Here we demonstrate that oxygen vacancies can be stabilized in the BiVO4 Scheelite structure through Sr2+ for Bi3+ substitution, leading to corner-sharing V2O7 tetrahedral dimers, and migrate via a cooperative mechanism involving V2O7-dimer breaking and reforming assisted by synergic rotation and deformation of neighboring VO4 tetrahedra. This finding reveals the ability of Scheelite structure to transport oxide ion through vacancies or interstitials, emphasizing the possibility to develop oxide-ion conductors with parallel vacancy and interstitial doping strategies within the same tetrahedra-based structure type.

The relationship between oxide-ion conductivity and cation vacancy order in the hybrid hexagonal perovskite Ba3VWO8.5

“…The relative energy barrier E1 is inuenced by the populations of the average O2/O3 sites, and its height decreases when the fractional occupancy of O3 increases, 26 since a larger number of lower coordination geometries provides more favourable and dynamic environments for oxygen transfer. 13,14,57 Fig. 7c shows that Ba 3 VWO 8.5 and Ba 3 NbMoO 8.5 have comparable relative BVSE energy barriers for the 2-dimensional ionic conduction along the palmierite-like layers, which are considerably smaller than the barrier for Ba 3 NbWO 8.5 .…”

High oxide ion and proton conductivity in a disordered hexagonal perovskite

“…However, YSZ shows adequate conductivities only at high working temperatures (> 700 °C), which poses technical challenges in terms of material selections and long-term durability, thus limiting the widespread application of SOFC technology 2,3,4 . Development of alternative electrolyte materials possessing good ionic conduction at intermediate temperatures (300 -600 °C) has led to the discovery of high oxide ion conductivity in several structural families 5,6,7,8,9,10,11,12 . High temperature proton conduction has been reported in doped barium cerates and zirconates, perovskite-related oxides, gallates and acceptor-doped orthoniobates and orthotantalates, which show proton conductivity when exposed to water vapour or hydrogen-rich atmospheres 10,13,14,15,16 .…”

“…The reduction in the number of higher coordination environments and subsequent increase in the concentration of lower coordination geometries must facilitate the ionic transport in the palmierite-like layers. It is well known that isolated tetrahedral/lower coordination moieties provide favourable and more dynamic environments for oxygen migration 10,11,12 . The presence of a flexible average tetrahedral environment facilitates proton mobility in Ba7Nb4MoO20, as previously reported for La0.8Ba1.2GaO3.9 10 , γ-Ba4Nb2O9 37 and the solid-acid protonic conductor CsH2PO4 38 .…”

Development of Melilite‐Type Oxide Ion Conductors

“…Even in isolated tetrahedral anion structures, oxygen vacancy-mediated oxide ion conductors are very few. [51,52] In 2017, Collins et al reported A-cation excess melilite structure on SrCa 1.5 Ga 3 O 7based compositions. [53] Therefore, the possibility that two La 3 + in the melilite structure are replaced by three Sr 2 + to form a solid solution of La 1-2/3x Sr 1 + x Ga 3 O 7 can not completely excluded for the nominal oxygen-deficient compositions La 1-…”