Oxygen incorporation in porous thin films of strontium doped lanthanum ferrite

Electrical conductivity relaxation measurements were carried out on thin films of (La(0.6)Sr(0.4))(0.99) FeO(3 -aEuro parts per thousand delta) deposited on MgO (100) substrates by pulsed laser deposition in order to determine the surface exchange coefficient, k (Ex), of the oxygen incorporation process in the temperature range 550-700A degrees C. The composition of the films was verified using wavelength dispersive x-ray and Rutherford backscattering spectroscopy. Scanning electron microscopy showed small triangular crystallites with the largest dimension 80 nm and the smallest dimension 10 nm. X-ray diffraction showed a cubic perovskite structure and significant texturing. At a constant temperature, k (Ex) was found to be a function only of the final pO(2) of the pO(2)-changes the sample was subjected to during conductivity relaxation experiments, confirming that the magnitude of the exchange coefficient was not influenced by changes in ionic defect concentrations. The k (Ex)-values determined for these thin films were significantly lower than for bulk samples. A value of 3.6 x 10(-6) cm s(-1)was obtained at 702A degrees C and a final pO(2), of 0.048 atm, approximately a factor of six lower than that obtained for bulk samples. An activation energy of 282 +/- 20 kJ mol (-aEuro parts per thousand 1) was found for the surface exchange coefficient at pO(2) = 0.048 atm. Possible reasons for the reduced magnitude of k (Ex) are discussed including the role of thermal history in influencing surface morphology and chemistry.