TITLE: Rare Earth Doped Complex Oxide Materials for Applications in Clean Energy ABSTRACT: There are many applications in the modern economy in which the adverse environmental affects can be mitigated by the utilization of ionic materials, such as in materials recycling, electricity generation, and energy efficiency. Specifically, complex oxides are increasingly being considered in such applications due to the ability to tailor their transport properties. The electronic, phonon, and ionic transport properties can be tuned for the intended application by fine adjustments in the composition of the material. We consider three examples of such applications: (a) The substitution of cerium into a structure known as sodium super ionic conductors (NaSICON, Na3Zr2Si2PO12). Tuning the composition and replacing the sodium with cerium results in an oxide material that has selective cerium ion mobility [(CexZr1-x)4/4-xNb(PO4)3], making it a candidate as the electrolyte in a real time cerium ion sensor. Similar sensors could be constructed to detect other rare earth elements, potentially reducing the cost and improving the efficiency of rare earth recycling process flows. (b) We next consider the application of lanthanum doped strontium manganese oxide as electronically conductive diffusion barriers. These barrier layers can increase the lifetime of SOFC stacks by reducing the high temperature oxidation of stainless steel interconnects in solid oxide fuel cells (SOFCs). This allows for cheaper interconnect materials to be used, further lowering the manufacturing cost. (c) As a final application we consider rare-earth doped strontium titanate as a thermoelectric material. We consider various strategies including the addition of a second phase of rare-earth doped cerium oxide to equilibrate the chemical potential of neutral oxygen within the material, thereby increasing the concentration of oxygen vacancies under reducing atmospheres, increasing the electronic conduction while simultaneously suppressing phonon transport, thereby increasing the overall figure of merit of the thermoelectric. COMMITTEE: Advisor: Srikanth Gopalan, MSE/ME; Xi Lin, MSE/ME; Aaron Schmidt, MSE/ME