Ellipsometric study of ferroelectric Ba_0.4Sr _0.6-xMn_xTiO₃ ceramics from 0.7 to 4.7 eV

Spectroscopic ellipsometry was used to extract the optical properties of Ba_0.4Sr_0.6-xMn_xTiO₃ (BSMT) (x from 1% to 20%) ceramics in the 0.7-4.7 eV (260-1700 nm) photon energy range at room temperature. X-ray diffraction analysis showed that BSMT ceramics are polycrystalline and lattice constants with different Mn composition present a slight variation. By reproducing the experimental ellipsometric spectra (Ψ and Δ), the optical constants and optical band gap energy have been obtained. It was found that the refractive index n increases first and then decreases as the photon energy increases from 0.7 to 4.7 eV for all the samples. The extinction coefficient k increases with increasing photon energy. On the other hand, both n and k decrease with increasing doping level of Mn (x ≤ 5%). Direct optical band gap energy is estimated to be 3.45-3.71 eV owing to different Mn doping. The difference of the optical properties can be ascribed to structure distortion with different Mn composition. The present results could be useful for future application of (Ba,Sr)TiO₃-based optoelectronic devices.