The effect of sub-surface strontium depletion on oxygen diffusion in La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ

The immediate surface and sub-surface composition of heat treated La0.6Sr0.4Co0.2Fe0.8O3−δ samples was measured by ion beam analysis and compared to oxygen transport properties over the same depth scale. Consistent with the literature, strontium segregation was observed for samples that received thermal treatments with the formation of an Sr–O based monolayer at the surface. Just below this, a sub-surface strontium depletion region over a depth scale of approximately 2–15 nm was seen. In this sub-surface region, a depletion of lanthanum was also observed. Oxygen transport properties were measured using isotopic labelling techniques and showed that, despite large changes in composition, the transport properties remain largely unchanged. Because strontium diffusion is slow in the bulk and grain boundaries only account for 0.03% of the material (due to large grains), it is suggested that 2D defects (such as dislocations and twins) can act as fast diffusion pathways for the strontium to account for the Sr depletion region observed below the surface.