Optical dispersion and band gap of pure and Mn‐doped 0.92Na0.5Bi0.5TiO3‐0.08K0.5Bi0.5TiO3 lead‐free single crystals

Optical properties of lead-free ferroelectric pure and Mn-doped 0.92Na(0.5)Bi(0.5)TiO(3)-0.08K(0.5)Bi(0.5)TiO(3) (NBT-8KBT) single crystals have been investigated systematically. Refractive index and extinction coefficient were measured and the critical parameters are obtained by modified Sellmeier dispersion equations and single-oscillator dispersion relation. The decline of refractive index for Mn:NBT-8KBT could be related to the lattice distortion of the Mn ions doping. High transmittance (>70%) over the transparent region (>400 nm) was found in the pure NBT-8KBT, higher than that of Mn:NBT-8KBT, which could be caused by the increase of domain wall density. An about 100 nm red-shift of absorption edge was observed in the Mn:NBT-8KBT single crystal, which is located in the visible region. Optical bandgap energies calculated through Tauc equation and a large bandgap difference of 2.96 and 2.62 eV occurs in the NBT-8KBT and Mn:NBT-8KBT, respectively, which illustrates the modulation of the band structures by Mn doping.