Solid solution limits and electrical properties of scheelite SryLa1-yNb1-xVxO4-δ materials for x = 0.25 and 0.30 as potential proton conducting ceramic electrolytes

Abstract

The proton conductivity and solid solubility limits of acceptor strontium doped vanadium stabilised lanthanum niobate (SryLa1−yNb1-xVxO4-δ, x = 0.25, 0.30 and y = 0 to 0.10) were explored as potential proton conducting ceramic electrolytes. All samples were synthesized via a solid-state method. The phase purity, microstructure and thermal expansion behaviour of the materials were studied using powder X-ray diffraction, scanning electron microscopy and dilatometry, respectively. A maximum solid solution limit of 5% Sr in the A-site of SryLa1−yNb1-xVxO4-δ samples is observed for a vanadium content of x = 0.25, while further increases in the Sr or vanadium contents lead to the presence of Sr3(VO4)2 as a secondary phase. This acceptor dopant content of 5%Sr in the current scheelite material exceeds that possible in the parent vanadium-free fergusonite SryLa1−yNbO4-δ material by a factor of 5. All Sr doped scheelite materials show linear thermal expansion behaviour, successfully avoiding the scheelite to fergusonite structural phase change during thermal cycling. The average grain size is shown to be increased by increasing vanadium content. In humid conditions, all phase pure samples show predominantly proton conductivity at lower temperatures, while p-type conductivity is noted at higher temperatures under dry oxidising conditions. In the low temperature range, the Sr0.05La0.95Nb0.75V0.25O4-δ sample, containing the largest acceptor dopant concentration, exhibits slightly higher bulk and specific grain boundary conductivities in comparison to other phase pure compositions.