Polaron formation and hopping in tantalate perovskite oxides: NaTaO3 and KTaO3

Abstract

Perovskite tantalates have become potential candidates for water splitting photocatalysts. Therefore, it is of importance to understand the behavior of the photoinduced excess charges in these materials. Herein, we investigate the formation of electron and hole polarons in ${\mathrm{NaTaO}}_{3}$ and ${\mathrm{KTaO}}_{3}$. We perform Perdew-Burke-Ernzerhof hybrid density functional $\mathrm{PBE}0(\ensuremath{\alpha})$ calculations, in which we define the fraction $\ensuremath{\alpha}$ of the Fock exchange by enforcing the Koopmans' condition, to properly account for self-interaction corrections in these calculations. We find that the hole polaron mainly localizes on one oxygen site in both materials, leading to a structural distortion where two Ta--O bonds are elongated. The electron polaron, on the other hand, localizes within one atomic plane and exhibits a two-dimensional electron gas nature. Finally, we find that the strong localization of holes leads to a low hole mobility at room temperature $\ensuremath{\sim}2.94\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}$/Vs and $\ensuremath{\sim}1.87\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}$/Vs for ${\mathrm{KTaO}}_{3}$ and ${\mathrm{NaTaO}}_{3}$, respectively.