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-xSixFe1-yTiyO3-delta 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(O-2)) range, 10(-20) to 0.5 atm. The total conductivity is predominantly p-type electronic in the P(02) 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) x 10(-6) K-1 at 300-900 K and (21.6-24.9) x 10(-6) K-1 at 900-1300 K in air. In oxidizing atmospheres the steady-state oxygen permeation fluxes through La1-xSrxFeO3-delta (x = 0.5-0.7) are similar, whereas under reducing conditions, La0.5Sr0.5FeO3-delta exhibits the highest ionic conductivity, rendering this composition a most promising parent compound for mixed-conducting membranes operating under large P(O-2) gradients. The activation energies for ionic transport at low p(O-2), 68-75 kJ/mol, are essentially independent of dopant concentration. (c) 2005 Elsevier SAS. All rights reserved.