Optical dispersion and bandgap of pure and Mn‐doped 0.92Na 0.5 Bi 0.5 TiO 3 ‐0.08K 0.5 Bi 0.5 TiO 3 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.