Transport properties and thermal expansion of Ti-substituted La1−xSrxFeO3−δ (x=0.5-0.7)

In order to identify the effects of minor titania additions and La:Sr cation ratio on the properties of lanthanum-strontium ferrites with high ionic conductivity, selected compositions in the La1−xSrxFe1−yTiyO3−δ system (x=0.5 and 0.7; y=0-0.25) have been appraised employing dilatometry, oxygen permeation studies and total conductivity with Seebeck coefficient measurements in the oxygen partial pressure (p(O2)) range, 10−20 to 0.5 atm. The total conductivity is predominantly p-type electronic in the p(O2) range 10−8-0.5 atm, with a relatively low, temperature-activated hole mobility indicating a hopping mechanism. Oxygen-ionic and n-type electronic transport is dominant for oxygen partial pressures below 10−14-10−10 atm. Titanium addition lowers electronic and oxygen ionic conductivity in both regimes, but also substantially decreases thermal expansion coefficients (TECs). The average TEC values are (12.4-13.6)×10−6K−1 at 300-900 K and (21.6-24.9)×10−6K−1 at 900-1300 K in air. In oxidizing atmospheres the steady-state oxygen permeation fluxes through La1−xSrxFeO3−δ (x=0.5-0.7) are similar, whereas under reducing conditions, La0.5Sr0.5FeO3−δ exhibits the highest ionic conductivity, rendering this composition a most promising parent compound for mixed-conducting membranes operating under large p(O2) gradients. The activation energies for ionic transport at low p(O2), 68-75 kJ/mol, are essentially independent of dopant concentration.