B-site substitutions in LaNb1−xMxO4−δ materials in the search for potential proton conductors (M=Ga, Ge, Si, B, Ti, Zr, P, Al)

The solid solubilities of potential B-site dopants in LaNb1-xMxO4−δ, materials, M=Ga, Ge, Si, Al, B, P, Zr or Ti, have been investigated in the search for possible novel proton conductors. In general, the solubility levels of these cations were found to be very low (x≤0.03). At the maximum value x=0.03, only compositions containing Ti, Ge, Ga and Si appeared pure at the limit of resolution of XRD. The literature phase diagram, La2O3–Nb2O5–ZrO2, has been re-analysed for compositions of low Zr-content around the composition LaNbO4. The electrical properties of phase pure Ti-doped compositions have been studied. Higher bulk and total conductivities were observed in wet than dry conditions, suggesting a significant protonic contribution to total conductivity. In wet conditions, the activation energy for bulk conductivity of LaNb0.98Ti0.02O4–δ was found to be much higher than that of an A-site, Sr-doped material, Sr0.02La0.98NbO4–δ, of similar acceptor dopant concentration. The Sr-doped composition offered higher conductivities than the Ti-doped composition up to approximately 900°C.

Comparison of total (open symbols) and bulk (closed symbols) conductivities measured in wet (p(H2O)≈0.026 atm.) nitrogen and oxygen atmospheres for LaNb0.98Ti0.02O4-δ (LNT2O) and Sr0.02La0.98NbO4-δ (S2LNO) materials.Figure optionsDownload as PowerPoint slideResearch highlights
► The article investigates solid solubilities of potential B-site dopants in the potential proton conductor LaNb1-xMxO4−δ,
► The literature phase diagram, La2O3–Nb2O5–ZrO2, has been corrected for compositions of low Zr-content around the composition LaNbO4.
► The electrical properties of Ti-doped compositions have been assessed in wet and dry conditions.
► Comparison has been made between the electrical properties of B-site, Ti-doped, compositions and A-site, Sr-doped, compositions.