Edge absorption spectra of crystalline and glassy PbGeS3

Abstract

Edge absorption spectra of crystalline and glassy lead thiogermanate (PbGeS3) have been measured in the temperature range from 77 to 470 K. It is shown that the dependence of the absorption coefficient on the photon energy for the glassy and crystalline states in the polarization E || c is described by the Urbach rule. For the crystal in the polarization E || b, at T < 300 K, an almost parallel shift of the intrinsic absorption edge to lower energies occurs with an increase in temperature, whereas at T ≥ 300 K, the Urbach absorption edge is observed. The parameter σ0, related to the electron–phonon coupling constant, and the energy ωph of the effective phonons involved in the formation of the absorption edge of crystalline PbGeS3 are determined from the temperature dependence of the parameter of the absorption edge slope. The contributions of the dynamic and static disorders to the diffusion of the absorption edge of crystalline PbGeS3, as well as the topological disorder of glassy PbGeS3, have been estimated.