Synthesis and examination of GdNb1-W O4+ new scheelite-type oxide-ion conductor

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

“…Fig. 13 illustrates σ b values acquired at 500 °C for different compounds featuring a scheelite-type structure (CeNbO 4+ δ , 10 GdNb 0.6 W 0.4 O 4.2 , 11 Pb 0.8 La 0.2 WO 4.1 , 16 Pb 0.8 La 0.2 MoO 4.1 , 17 LaNb 0.92 W 0.08 O 4.04 , 12 LaNb 0.8 Mo 0.2 O 4.1 , 14 Ca 0.9 Cs 0.1 WO 3.95 , 18 Ca 0.9 K 0.1 WO 3.95 , 19 Bi 0.95 Sr 0.05 VO 3.975 , 7 and Bi 0.85 Ca 0.15 VO 3.925 ). The values range from 2 × 10 −6 for GdNb 0.6 W 0.4 O 4.2 to 7 × 10 −3 S cm −1 for CeNbO 4+ δ .…”

Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms

“…The proton defects are created by absorbed water molecules filling the oxygen vacancies according to the equation H 2 O + V O •• + O O ↔ 2OH O • ,,,, In contrast with the BiVO 4 , the oxygen vacancies in the A NbO 4 structure did not prompt oxide ion conduction but facilitated the stabilization and migration of protons, leading to impressive proton conductivities (10 –3 S·cm –1 on La 0.99 Ca 0.01 ­NbO 3.995 composition at 800 °C). − Ferrara et al ,, used atomistic static lattice simulations and DFT calculations to investigate the hydration and proton diffusion in scheelite LaNbO 4 -based proton conductors, which revealed that the proton defects were located in the position adjacent to lanthanum cations and the center between NbO 4 tetrahedra and then migrate via rapid exchange between the oxygen ions within the same tetrahedron with a barrier energy of ∼0.05 eV and jumping between two adjacent tetrahedra with an activation energy of ∼0.4 eV. Under the dry atmosphere conditions, the corner-sharing tetrahedral units could be present in the acceptor-doped A NbO 4 , as implied by the BiVO 4 case.…”

“…Since then, more and more scheelite-type oxide conductors based on compounds with the general formula AM O 4 ( A = alkaline-earth metals, rare-earth metals, Pb 2+ , and Bi 3+ ; M = W 6+ , Mo 6+ , V 5+ , Nb 5+ , etc.) have been reported, in which some of them reached optimal oxide ion conductivities as high as 10 –3 S·cm –1 at 500 °C. ,− …”

“…The scheelite structure possesses high rotation and deformation flexibility thanks to the presence of isolated M O 4 units that can adapt variable coordination numbers (4 to 6) of M cations, facilitating stabilization and migration of the oxygen defects. Unlike the apatite- and melilite-type oxide ion conductors, the scheelite structure can accommodate not only an excess of oxygen ions as interstitial defects, e.g., donor-doped A NbO 4 ( A = lanthanide), ,,,, but also oxygen vacancies, e.g., acceptor-doped BiVO 4 , ,, with high oxide ion mobilities for both charge carrier types. Nevertheless, scheelites-type structures usually display oxygen-hyperstoichiometric compositions, and the oxygen-deficient scheelite compositions are scarce.…”

Electrochemical Properties of Cs and La Co-doped CaWO₄ Oxide Ion Conductor

“…[3][4][5] Similar enhancement in ionic conduction due to the oxide ion interstitials has been reported for other scheelite-type structured systems, such as PbMoO 4 , 6 CaMoO 4 , 7 BaMoO 4 , 8 high-temperature form of CeNbO 4 , 9 LaNbO 4 , 10,11 or GdNbO 4 . 12 However, CaWO 4 with mineral name of scheelite is unlikely to form solid solution by lanthanum substitution incorporating excess oxide ions at interstitial site. 1 We have recently reported that monovalent ion can be substituted into CaWO 4 forming oxide ion vacancy as Ca 1-x A x WO 4-x/2 (A = K or Cs), which also shows high oxide ion conduction at elevated temperature.…”