Electrical conductivity, oxygen diffusion and surface exchange studies on a melilite-type La1.05Sr0.95Ga3O7+δ

An oxide ion conducting lanthanum excess lanthanum strontium trigallium heptoxide, La1+xSr1-xGa3O7+δ (x=0.05) was prepared by the high temperature ceramic route at 1350 °C in air and characterized by powder X-ray diffraction (XRD), energy-dispersive analysis by X-ray (EDAX) and thermogravimetric techniques to establish the phase purity, composition and amount of oxygen excess respectively. The value of ‘δ’ was found to be 0.025. The a and c parameters were evaluated from the high temperature XRD patterns and indicated non-linear behaviour with a gradual increase up to 600 °C and a steep increase thereafter. The thermal expansion coefficient (α) was calculated to be 8.285×10−6/K valid in the temperature range of 35–1000 °C. Electrical property measurements were carried out on 96% dense pellets and the bulk conductivity of La1.05Sr0.95Ga3O7.025 obtained from AC impedance studies was approximately two and three orders of magnitude less than YSZ and LSGM, respectively. Oxygen diffusion data were obtained by isotopic anneals followed by SIMS line scanning. The tracer diffusion coefficient (D*) and surface exchange coefficient (k*) calculated from the diffusion profile increased with increasing temperature. A reasonable agreement was established between the measured bulk conductivity and the contribution to conductivity from the oxide ions demonstrating oxide ion conduction.