Effect of Polaron Formation on Carrier Transport Properties of Transition Metal Oxides

Transition metal oxides (TMO) are promising candidates for materials in energy conversion and storage applications. However, due to formation of small polarons, low electrical conductivity has been the major bottleneck for its practical applications. Atomic doping has been shown as an effective approach to improve their electrical conductivity, but a detailed mechanistic understanding is still needed and important for experimental synthesis of ideal materials. Since atomic doping affects both carrier concentration and carrier mobility, they are both important for understanding dopant effects on electrical conductivity. Using hematite as a prototypical TMO, we have shown an accurate prediction of carrier concentration through our recently developed method of calculating carrier concentration based on charge neutrality condition, and prediction of polaron mobility of doped systems through combining generalized Landau-Zener theory with kinetic Monte-Carlo sampling. With both quantities, we then calculated carrier conductivity in good agreement with experiments.

In particular, we demonstrate the critical role of synthesis temperature and oxygen partial pressure on determining dominant defects, dopant solubility, and polaron concentrations through our calculations. Raising synthesis temperature or lower oxygen partial pressure is shown to be promising pathway for overcoming doping bottleneck. Furthermore, we reveal the trend of dopant effect on polaron mobility in Fe₂O₃ could be explained by Fe sublattice disorder described by Fe-Fe pair distribution function, where stronger disorder increases the effective hopping barrier and lowers the polaron mobility. We confirm

the local structure from our calculation by remarkable agreement with experimental EXAFS measurements. Through both polaron mobility and concentration calculations from first principles, we provide important mechanistic insight into the effect of dopants on electrical transport in polaronic oxides.