Synthesis and characterisation of vanadium doped alkaline earth lanthanum germanate oxyapatite electrolyte

The crystal chemistry and oxygen conductivity of vanadium-doped apatites of nominal composition [La ₈AE ₂][Ge _6-xV _x]O _26+x/2 (AE = Ca, Sr, Ba; 0 ≤ x ≤ 1.5) were studied to establish their potential as intermediate temperature solid oxide fuel cell (SOFC) electrolytes. Single-phase products obtained for x ≤ 0.5 were found, using a combination of powder synchrotron X-ray and neutron diffraction, to be P6 ₃/m apatites. The ionic conductivities extracted by complex impedance spectroscopy showed that small vanadium amendments enhanced oxygen mobility at intermediate temperatures (500-700 °C) by more than one order of magnitude, as the incorporation of V ⁵⁺ through displacement of Ge ⁴⁺ is charge balanced with interstitial O ^2- that improves ionic conduction. The most promising composition was La _7.88Ca ₂Ge _5.35V _0.65O _26.15 that delivered σ = 3.44 × 10 ^-4 S cm ^-1 at 500 °C. The superstoichiometric oxygen was delocalised, without fixed X-ray or neutron scattering centres. Crystal chemistry systematics demonstrate that the Ca-apatite was superior because the relatively small framework expanded the tunnel through reduced (La/AE)O ₆ metaprism twisting (φ), ensuring the P6 ₃/m symmetry was adopted, which favours the passage of O ^2- with lower activation energy.