Insight into the synthesis and electrical properties of alkali-earth-substituted Gd₃GaO₆ oxide-ion and proton conductors

Novel ionic conductors were prepared by substituting Ca²⁺ and Sr²⁺ for Gd³⁺ in Gd₃GaO₆. A microwave-assisted combustion technique was used to synthesize these compounds at 900 °C. SEM observations showed that both substituents promote grain growth during sintering. XRD proved that the Gd_3-x(Ca,Sr)_xGaO_6-x/2 solid solutions are formed up to x = 0.10. Below 600 °C, the level of conductivity under wet Ar is higher than that of measured under dry atmospheres, thereby demonstrating the contribution of proton defects to the overall conductivity. The highest level of proton conduction, i.e. σ_600°C = 1 × 10⁻³ S cm⁻¹, was measured for Gd_2.9Sr_0.1GaO_5.95 at 600 °C in wet Ar. At higher temperatures, only oxygen ions contribute to the conductivity. At 800 °C, a total oxide-ion conductivity of σ_800°C = 1 × 10⁻² S cm⁻¹ was measured for the highest substitution level, i.e. x = 0.10. In both temperature ranges, activation energy associated with ionic transport decreases with the Me content as a result of an increase in grain size. Stability tests were successfully achieved as the structure of materials remains unchanged after different treatment under severe conditions. Conductivity measurements under varying oxygen partial pressures demonstrated that materials are purely oxide-ion conductors up to pO₂ = 1 × 10⁻⁵ atm. At higher pO₂, a p-type contribution appears.