Modeling mixed conductivity in solid-state electrolytes
The overarching goal of the the first-principles computational studies presented in this dissertation is to better understand and predict charge carrier interactions in mixed conductors. Rare-earth phosphates and hematite are chosen as example insulators which show reasonably high proton and polaron conductivity, respectively. Rare-earth phosphates are not yet used commercially, but show promise as intermediate temperature fuel-cell electrolytes, so a first-prinicples atomic-level understanding of transport in these materials will enable engineering of their properties. Due to close collaborations with experimentalists working in synthesis and characterization of rare-earth phosphates via impedance, X-ray photoemission, and NMR spectroscopy, the density functional theory (DFT) studies are compared directly to experimental measurements.
Significant charge carrier concentrations are created in these materials by aliovalent doping, which can significantly increase the activation energy barrier for transport through the material. In this thesis, the interaction of protons and Ba-dopants in LaPO4 is investigated and the X-ray photoemission spectrum of CePO4 is interpreted via its calculated electronic structure, showing that hole small-polarons on Ce ions are stable defects. Finally, a new method for calculating small-polaron mobilities in crystals with DFT plus a Hubbard-U term is presented and found to work well for electron small-polaron mobilities in hematite.