Fundamental optical absorption edge and exciton–phonon interaction in Cu6PS5Br superionic ferroelastic

The absorption edge of monocrystalline Cu6PS5Br was studied in the 4.2–505 K temperature range. At low temperatures (T<180 K), Wannier–Mott s-excitons were detected at the absorption edge as well as in the photoluminescence spectra and, their basic parameters at 77 K were estimated. The mechanisms of exciton–photon interaction resulting in exponential absorption edge and Urbachʹs tail at temperatures T>200 K were analyzed and interpreted in the framework of the Dow–Redfield theory. Two types of structural disorder affecting the optical absorption tail were studied.