Proton conductivity of hexagonal and cubic BaTi1−xScxO3−δ (0.1 ≤ x ≤ 0.8)

Abstract

BaTi_1−xSc_xO_3−δ (x = 0.1–0.8) was prepared via solid state reaction. High resolution X-ray powder diffraction was used to characterise the synthesised materials. It was found that low substitution (x = 0.1 and 0.2) of Ti⁴⁺ for Sc³⁺ gives a hexagonal perovskite structure, whereas high substitution (x = 0.5–0.7) results in a cubic perovskite structure. Thermogravimetric analysis revealed significant levels of protons in both as-prepared and hydrated samples. Electrical conductivity was measured by AC impedance methods under oxygen, argon and under dry and humid, both H₂O and D₂O, conditions for BaTi_1−xSc_xO_3−δ (x = 0.2, 0.6 and 0.7). In the temperature range of 150–600 °C, under humid conditions, the conductivity is significantly higher than that under the dry conditions. The increase in conductivity is especially prominent for the cubic phases, indicating that protons are the dominant charge carriers. The proton conductivity of hexagonal BaTi_0.8Sc_0.2O_3−δ is approx. two orders of magnitude lower than that of the more heavily substituted cubic phases. Conductivity is also found to be higher in dry O₂ than in Ar in the whole temperature range of 150–1000 °C, characteristic of a significant contribution from p-type charge carriers under oxidising atmospheres. Greater Sc³⁺ substitution leads to a higher proton concentration and the highest proton conductivity (σ ∼ 2 × 10⁻³ S cm⁻¹ at 600 °C) is found for the BaTi_0.3Sc_0.7O_3−δ composition.