Dielectric functions of ferroelectric Pb_0.5Sr _0.5TiO₃ film determined by transmittance spectroscopy

Pb_0.5Sr_0.5TiO₃ (PST) films having a pure polycrystalline perovskite phase and well-grown grains were prepared by a chemical solution deposition method. Their optical response from 1.1 to 5.0 eV was investigated by transmittance spectroscopy, and the complex dielectric functions and optical parameters were evaluated in terms of the Tauc-Lorentz and Cauchy models. The direct optical band energy of the interband transition from the valence to conduction bands was determined to be 4.09 eV. Below this fundamental absorption region, the optical response in the energy range of 3-4 eV behaves as a local-states-related Urbach-band-tail absorption with the tail energy of about 243 meV. By fitting the refractive index using a single oscillator model, it is found that the optical dispersion of the PST films in the transparent region (1-3 eV) may originate mainly from the direct interband critical point transition between the valence and conduction bands.