Oxide ion conduction in Ba, Ca and Sr doped apatite-type lanthanum silicates

Abstract

The apatite-type compounds La_9.33□_0.67Si₆O₂₆, La_{10 − x}Ba_x(SiO₄)₆O_{2 + δ} with x = 0.5 → 2, La₉Ca₁(SiO₄)₆O_{2 + δ} and La₉Sr₁Si₆(SiO₄)₆O_{2 + δ} were prepared by high temperature solid state reaction with a high purity level. The La₉Ba₁Si₆O_26.5 composition is a good electrolyte for SOFCs at intermediate temperature, with a pure ionic conductivity of 1.16 × 10⁻² S cm⁻¹ at 700 °C and low activation energy of 0.74 eV. Moreover the conductivity is stable after one month of ageing at 900 °C under air or reducing atmosphere and is increased with the ceramic grain size. The comparison of undoped La_9.33□_0.67Si₆O₂₆ apatite and doped La₉AE₁(SiO₄)₆O₂₆ apatites with AE= Ca, Sr, Ba shows an increase in conductivity with the doping concentration. For doped apatites, the conductivities and activation energy are related to the dopant size, the best properties being obtained for the barium doping. The existence of some extra oxide ions was demonstrated by NPD experiments for oxygen excess apatites but there was no evidence for a new crystallographic interstitial oxygen site in the hexagonal channel. The oxide ion O(5) in the hexagonal channel exhibits a large spread of the nuclear density along the c-axis in all samples, probably due to a static disorder. The cooperative displacements of the SiO₄ tetrahedra and of the La(3) site towards the O(5) channel suggest a conduction mechanism involving mainly the extra oxide ions with a migration path between the O(5) channel and the SiO₄ tetrahedra along the [001] direction.