Structure and stability of NaTa O3(001) and KTa O3(001) surfaces

We carried out first-principles slab calculations to investigate the structure, energetics, and electronic properties of the majority (001) surfaces of NaTaO3 (NTO), a perovskite oxide with excellent photocatalytic properties, and KTaO3 (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 TaO2 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 TaO2 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 Na+ relative to K+. Upon exposure to water, a (2 × 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.