Ionic conductivity of SrO-Ta2O5 and SrO-La2O3-Ta2O5 compounds

Abstract

The conductivities of Sr₂Ta₂O₇, Sr₃TaO_5.5, Sr₂LaTaO₆ and Sr₃LaTa₃O₁₂, both doped and undoped, were measured by the ac complex impedance technique over a temperature range of approximately 400–800 °C. The conductivity of Sr₂Zr_2xTa_{2 – 2x}O_{7 –x} at 800 °C increased from 5.4 × 10^–6 S cm^–1 for the undoped compound to 3.0 × 10^–4 S cm^–1 at ×= 0.1, and the activation energy correspondingly decreased from 0.98 to 0.63 eV. Sr₃TaO_5.5 and Sr₂LaTaO₆ were found to be the endpoints of a solid solution which can be described by Sr_{2 +x}La_{1 –x}TaO_{6 –x/2} where x ranges from 0 to 1. It has the (NH₄)₃FeF₆-type fee structure. There was a maximum in conductivity at x = 0.75 with an oxygen vacancy concentration of 6.25%. The corresponding conductivity at 800 °C was 6.2 × 10^–3 S cm^–1, nearly identical to 15 % CaO-stabilized ZrO₂. The conductivities at 800 °C of Sr₂LaTaO₆ and Sr₃LaTa₃O₁₂ were 3.6 × 10^–5 and 1.0 × 10^–6 S cm^–1, and the activation energies over the 400–800°C temperature range were 1.41 and 0.84 eV, respectively.