Using thin films to investigate heterogeneous defect chemistry

Prof. Nowick showed that the controlling mechanism of ion conduction in traditional solid electrolytes can be effectively summarized with a model built on defect chemistry within an infinite crystalline lattice. A common assumption in these models has been that, at least at low concentrations, dopants are randomly scattered within their particular sublattice. More recently, experimental tools have established means to create mesoscale compositional heterogeneity. This capability allows significant extension of the experimental space beyond what can be captured with the traditional models. Here, we survey recent experimentation that uses a sputtering technique to create films with composition Ce1-x-z Zr x D z O2-z/2 (D=Y, Gd, or La) where x or z can vary through the thickness of the film at the single nanometer level. These films are used to study 1) the effect of lattice mismatch strain by modulating the Ce/Zr ratio in multilayers and 2) the effects of vacancies being trapped within planar space charge regions by locating the dopant atoms as 2-D sheets within otherwise pure CeO2 films. The films are likely to be metastable, but maintain compositional heterogeneity over experimental time scales. Current results and future possibilities for this technique are discussed.