Magnetic, optical, and electron transport properties ofn-type CeO2: Small polarons versus Anderson localization

We report magnetic susceptibility, electrical conductivity and optical absorption of ${\mathrm{Ce}}_{1\ensuremath{-}x}{M}_x{\mathrm{O}}₂$ where $M$ = Nb,Ta and $0\ensuremath{\le}x\ensuremath{\le}0.03$. The dc conductivity follows a simple thermally activated Arrhenius-type behavior in the $T=70$--700 K range with a change in slope at ${T}^*\ensuremath{\approx}155$ K. The high-temperature activation energy shows gradual increase from $\ensuremath{\approx}170$ to 220 meV as the dopant concentration increases. The activation energy of the low-temperature conductivity shows a broad minimum of $\ensuremath{\approx}77$ meV at $x\ensuremath{\approx}0.01$. Electron transport and localization mechanisms are analyzed in the framework of the Holstein small polaron, Anderson localization, and Jahn-Teller distortion models. The fit to the small polaron mobility is dramatically improved when, instead of the longitudinal phonons, the transverse optical phonons are considered in the phonon-assisted electron transport. This serves as an indirect evidence of a strong $4{f}¹$ orbital interaction with the oxygen ligands, similar to the case of ${\mathrm{PrO}}₂$. Based on comparison of the experimental data to the models, it is proposed that the defect-induced random electric fields make the dominant contribution to the electron localization in donor-doped ceria.