Structure and stability of NaTaO3(001) and KTaO3(001) surfaces

We carried out first-principles slab calculations to investigate the structure, energetics, and electronic properties of the majority (001) surfaces of $\mathrm{NaTa}{\mathrm{O}}_{3}$ (NTO), a perovskite oxide with excellent photocatalytic properties, and $\mathrm{KTa}{\mathrm{O}}_{3}$ (KTO), a closely related but somewhat less active compound. Being polar, NTO(001) and KTO (001) require charge compensation to be stabilized. We examine a number of possible structural models for these surfaces by comparing their formation energies to those of the pure NaO/KO and $\mathrm{Ta}{\mathrm{O}}_{2}$ terminations. Our results show that a ``cation-exchange'' reconstruction is energetically most favorable for NTO(001) under vacuum conditions, whereas for KTO(001) this reconstruction competes with a ``striped'' phase with equally exposed KO and $\mathrm{Ta}{\mathrm{O}}_{2}$ terraces actually observed in recent experiments. NTO is found to exhibit enhanced structural flexibility and more effective charge compensation in comparison to KTO, which is attributed to the significantly smaller size of $\mathrm{N}{\mathrm{a}}^{+}$ relative to ${\mathrm{K}}^{+}$. Upon exposure to water, a (2 \ifmmode\times\else\texttimes\fi{} 1) hydroxylated structure is by far most favorable for both NTO and KTO. This structure can thus provide a basis for the mechanistic understanding of photocatalytic processes on NTO and KTO surfaces.