Low Energy Ion Scattering (LEIS) Applied to Ionic Materials

Surfaces and interfaces play a dominant role in the performance of devices like fuel cells and membranes based on ionic materials. The LEIS technique offers a unique possibility to determine the composition of the outermost atomic layer, which is often quite different from that of deeper layers. For instance impurities that are present in minute concentrations -1 ppb or even less-in the bulk may segregate to the surface and there become the dominant species. By carefully sputtering away the top layers also a compositional depth profile can be made of the first 10 to 20 layers. The basic principles of the LEIS technique will be explained, answering questions like: why is it so extremely surface sensitive; What does the instrumentation look like; what can you learn from the results. Its applicability to ionic materials will be demonstrated with results from measurements on various substrates used in fuel cells. The oxygen exchange across the surface of Sm(0.8)Sr(0.2)CoO(3) can be studied, because it is possible to differentiate between the (18)O and (16)O isotopes. One can determine which elements are on top in La(0.8)Sr(0.2)Ga(0.8)Mg(0.2)O(3 +/-delta). The top layer of Ce(0.8)Gd(0.2)O(1.9) proves to be enriched in Gd oxide. The Y segregation in isotopically enriched YSZ is being studied.