Oxygen tracer diffusion and surface exchange kinetics in La0.6Sr0.4CoO3 − δ
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A.V. Berenova, , , A. Atkinsona, J.A. Kilnera, E. Bucherb and W. Sitteb
a Department of Materials, Imperial College London, London, SW7 2AZ, UK
b Chair of Physical Chemistry, University of Leoben, 8700 Leoben, Austria
Received 20 January 2010;
revised 26 April 2010;
accepted 26 April 2010.
Available online 31 May 2010.
Abstract
The oxygen tracer diffusion coefficient, D*, and the oxygen tracer surface exchange coefficient, k*, were measured in La0.6Sr0.4CoO3 − δ over the temperature range of 406–680 °C and the oxygen partial pressure range 0.044–1.019 bar. Distortions of the near surface regions of the diffusion profiles were observed in several specimens. 18O exchange experiments were modeled by the Finite Element method and the origin of the observed distortions was assigned to exchange of oxygen during sample cooling. The estimated error in the determination of D* and k* was found to be less than 20%. The activation energies of D* and k* were 1.84 ± 0.02 eV and 0.76 ± 0.13 eV, respectively. D* was found to decrease with oxygen partial pressure following the power law with a power of − 0.55 ± 0.10. The effect of Sr doping on the values of D* in Ln1 − xSrxCoO3 − δ (Ln = La, Sm) perovskites is discussed. The data obtained from isotopic exchange measurements at 500–600 °C are in good agreement with the kinetic parameters derived from conductivity relaxation experiments on samples from the same batch.
Fig. 1. FIB image of La0.6Sr0.4CoO3 − δ specimen. The vertical scale is different from the horizontal one as the sample was tilted to 45°. The specimen was annealed at 600 °C for 349 min. Inset shows the effect of sputtering conditions (sputtering time, current, and sputtered area) on the depth of the crater sputtered with a 5 keV Ar beam.
Fig. 3. 18O depth profiles in La0.6Sr0.4CoO3 − δ exchanged at 600 °C for 15 min (a) and after re-exchange under conditions similar to the first exchange (b).
Fig. 4. Deviation of the 18O diffusion profiles modelled by Finite Element method, C(x), from those calculated from Eq. (1) without taking into account sample heating/cooling, C′(x) as a function of depth.
Fig. 6. Temperature dependence of the oxygen tracer diffusion coefficient in Ln0.6Sr0.4CoO3 − δ (Ln = La, Sm). Solid line is a guide to eye. IEDP — isotope exchange depth profiling, GPA —gas phase analysis, CR— conductivity relaxation techniques. The oxygen partial presures during the experiments were 0.97 atm [10], 0.21 atm (this study, [2]) and 0.023 atm [21].
Fig. 7. Temperature dependence of the oxygen tracer surface exchange coefficient in Ln0.6Sr0.4CoO3 − δ (Ln = La, Sm). Solid line is a guide to eye. The oxygen partial presures during the experiments were 0.97 atm [10] and 0.21 atm (this study).
Fig. 8. Effect of oxygen partial pressure on the oxygen tracer diffusion coefficient in La0.6Sr0.4CoO3 − δ at 600 °C. The oxygen non-stoichiometry data in La0.6Sr0.4CoO3 − δ at 725 °C (closed triangles, [34]) and 715 °C (open triangles, [21]) are also given.
Fig. 10. Effect of Sr doping in Ln1 − xSrxCoO3 − δ (Ln = La, Sm) on the values of D* and δ/3 at 600 °C (a) and 800 °C (b). The values marked by asterisks were estimated from higher temperatures. The non-stoichiometry data of La0.6Sr0.4CoO3 − δ were measured at 0.21 atm (closed triangles [33], open triangles [34]).
Fig. 12. Effect of Sr doping in Ln1 − xSrxCoO3 − δ (Ln = La, Sm) on the values of k* at 600 °C (a) and the activation energy of k* (b). The values marked by asterisks were estimated from higher temperatures.
The values of D* and k* used in the FEM and the values of D* and k* calculated from Eq. (1), using the FEM generated data after the 1 μm layer close to the surface was discarded. Heating and cooling rate was 3 K/s.
