Alkali metal-substituted bismuth-based perovskite compounds: A DFT study

Abstract

In this study, we have performed density functional theory based calculations to investigate the effect of alkali substituents M on the structure and electronic properties of (Bi0.5M0.5)TiO3 compounds with M = Li, Na and K. When the ionic radius of the substituting M cation increases, the corresponding direct band gap of (Bi0.5M0.5)TiO3 also increases. The same trend is found for the O-deficient Bi0.5M0.5TiO3-δ compounds, which were investigated by removing one Oxygen atom from the super cell creating an O-vacancy in it. The presence of the O-vacancy breaks the local symmetry of the crystal structures and generates excess electrons in the O-deficient compounds. This results in the appearance of an indirect bandgap in Bi0.5Na0.5TiO3-δ and Bi0.5K0.5TiO3-δ, except for Bi0.5Li0.5TiO3-δ that still exhibits a direct bandgap. Excess electrons induce the defect states within the original bandgap of the stoichiometric compounds and decrease the bandgap of the O-deficient compounds. In line with the electronic calculations, effective mass calculations have provided a preliminary insight into the photocatalytic performance of (Bi0.5M0.5)TiO3 and reveal a positive impact of K cations on the photocatalytic activity.