First-principles investigation on the structural, elastic, electronic and optical properties and possible mechanism of the photocatalytic properties for orthorhombic and tetragonal KNbO3

The structural, elastic, electronic, optical and photocatalytic properties are studied for orthorhombic and tetragonal KNbO3 (KNO) based on first-principles methods. The elastic results show that both phases are mechanically stable, and the tetragonal phase exhibits higher hardness and stiffness than the orthorhombic one. The elastic properties also exhibit obvious elastic anisotropy in both crystals. The electronic structures with HSE06 scheme show that both systems are indirect band gap semiconductors, and the Nb-O bonds in tetragonal phase have stronger covalency than those in orthorhombic phase. The optical properties are also calculated with HSE06 method. The complex dielectric function and linear optical properties are discussed in detail. The significant optical anisotropy is observed in (001) direction of tetragonal phase. Then the mechanisms of photocatalytic properties of both polymorphs are further discussed and compared. From the present band structures, the effective mass of photogenerated carriers and optical absorption spectra results, the photocatalytic activity of orthorhombic phase can be higher than the tetragonal one.