Advanced Physical Research

 The spectroscopic ellipsometric measurements of AgSbTe₂ were carried out. The spectra of optical constants and dielectric permittivity were determined in the 0.042-6.495 eV photon energy range. In order to determine the energies of optical interband transitions, the band-gap energy, and the values of physical parameters describing the optical constant spectra (ε₁, ε₂, n, k, α), the Tauc, Sokolov, Tauc–Lorentz (TL), Cody-Lorentz (CL), and modified Forouhi–Bloomer (mFB) models were used. Significant sub-gap Urbach absorption, caused by strong structural disorder, was observed. The optical constant spectra were parameterized using the TL, CL, and mFB models while considering the influence of Urbach absorption. The calculated and fitted optical constant spectra demonstrated excellent agreement with the experimental data. The obtained bandgap values were E_g =0.154-0.161  eV (for TL and CL models), and E_g = 0.215-0.220 eV (for mFB and Tauc models), respectively. It was shown that the Cody–Lorentz model provides a superior description of the sub-gap region of the optical constant spectra. The spatial structural disorder strongly influences the shape of the absorption edge, the optical properties, and the bandgap structure of AgSbTe₂ crystals.