Effect of In-Doping on Electronic Structure and Optical Properties of Sr₂TiO₄

The effect of In doping on the electronic structure and optical properties of Sr₂TiO₄ is investigated by a first-principles calculation of plane wave ultrasoft pseudopotentials based on density functional theory. The calculated results reveal that corner-shared TiO₆ octahedra dominate the main electronic properties of Sr₂TiO₄ and the covalency of the Ti-O(1) bond in the ab plane is stronger than that of the Ti-O(2) bond along the c-axis. After In doping, there is a little lattice expansion in Sr₂In_0.125Ti_0.875O₄ and the interaction between the Ti-O bond near the impurity In atom is weakened. The binding energies of Sr₂TiO₄ and Sr₂In_0.125Ti_0.875O₄ estimated from the electronic structure calculations indicate that the crystal structure of Sr₂In_0.125Ti_0.875O₄ is still stable after doping, but its stability is lower than that of undoped Sr₂TiO₄. Moreover, the valence bands (VBs) of the Sr₂In_0.125Ti_0.875O₄ system consist of O 2p and In 4d states, and the mixing of O 2p and In 4d states makes the top VBs shift significantly to high energies, resulting in visible light absorption. The adsorption of visible light is of practical importance for the application of Sr₂TiO₄ as a photocatalyst.