Oxygen incorporation in porous thin films of strontium doped lanthanum ferrite

Abstract

Electrical conductivity relaxation measurements were carried out on thin films of (La0.6Sr0.4)0.99 FeO3 − δ 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 550–700°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 $p_{\mathrm{O_{2}}}$ of the $p_{\mathrm{O_{2}}}$ -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 × 10 − 6 cm s − 1 was obtained at 702°C and a final $p_{\mathrm{O_{2}}}$ of 0.048 atm, approximately a factor of six lower than that obtained for bulk samples. An activation energy of 282 ± 20 kJ mol − 1 was found for the surface exchange coefficient at $p_{\mathrm{O_{2}}}$ = 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.