Exploring the nature of the fergusonite–scheelite phase transition and ionic conductivity enhancement by Mo⁶⁺doping in LaNbO₄

Abstract

Experimental and theoretical evidence points to first-order character of the fergusonite–scheelite transition in LaNbO4, and dopant Mo6+ and interstitial O2− defects exhibit competing effects on local oxide ion mobility.

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 .…”

“…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)).…”

Solid oxide proton conductors beyond perovskites

“…The fergusonite-scheelite transition in LaNbO4 has originally been described as a second order transition between the ferroelastic monoclinic phase and the paraelastic tetragonal phase 143 . However, recent reports suggest a reconstructive first order transition induced by displacement of the Nb cation from the centre of the tetrahedron and change in NbO coordination from 4 in the scheelite phase to a 6-coordinated distorted octahedral arrangement in the fergusonite phase, with two long and four short Nb-O distances 144,145 . The fergusonite-scheelite transition is relevant to the proton conductivity, since it coincides with a reduction in the activation energy for proton transport (from 0.78 eV to 0.55 eV in La0.99Ca0.01NbO4-δ), which This journal is © The Royal Society of Chemistry 20xx…”

Defects and disorder in apatite-type silicate oxide ion conductors: implications for conductivity

“…The development of solid oxide ion conductors for use as electrolytes in solid oxide fuel cells relies on improving the conductivity of the materials by design, to make them suitable for lower operating temperatures. Tetrahedrally coordinated cations have been identied as structural motifs that can contribute to the oxide ion conductivity mechanisms in materials belonging to different structural families, including brownmillerites, [1][2][3] melilites, 4,5 d-Bi 2 O 3 derivatives, 6-8 A 3 OhTd 2 O 7.5 compounds, 9,10 apatites, 11-13 scheelites [14][15][16] and hexagonal perovskites. 17,18 La-containing apatite-type silicate materials, La 9.33+x (SiO 4 ) 6 -O 26+1.5x (0 # x # 0.67), are particularly interesting owing to their high thermal and chemical stability, low toxicity and low cost.…”