Ellipsometric study of near band gap optical properties of SrxBa1−xNb2O6 crystals

Complex refractive indices N = n + ik for a-cut SrxBa1xNb2O6 single crystals with four nominal values of composition x = 0.40; 0.50; 0.61; 0.75 were studied by spectroscopic ellipsometry in the photon energy range 2–5 eV of crystal transparency and fundamental electronic absorption. The ellipsometric data were evaluated taking into account an uniaxial anisotropy and an unintentional surface roughness. We use a global composition dependent Sellmeier fit as a reference for monitoring the crystals composition. The optical model yields indirect and direct optical band gap energies Eg in the spectral range 3.77–4.03 eV and 4.28–4.55 eV, respectively, depending on the Sr/(Ba + Sr) content ratio x and light polarization. The congruently melting composition Sr0.61Ba0.39Nb2O6 shows distinct dispersion features in the imaginary part of the UV complex refractive index with respect to other compositions.