The excitonic processes and Urbach rule in Cu6P(S1-xSex)5I crystals in the sulfur-rich region

The temperature behavior (4.2–320 K) of the fundamental absorption edge of Cu6P(S1-xSex)5I mixed crystals (x = 0.05, 0.1, 0.15, 0.2) was studied. Bound and free direct excitons were detected at low temperatures in the dielectric phase (T<Ts) for x = 0.05 and x = 0.1 crystals, at the absorption edge and in the photoluminescence spectra. The spectral and temperature behavior of the long-wavelength side of the absorption edge in the superionic phase is described by the Urbach rule. Mechanisms of the exciton-phonon interaction, resulting in an exponential Urbach edge, were analyzed taking into account various types of disorder. Increasing of the selenium content results in an increase in the energy width of the Urbach absorption edge, a decrease of the optical bandgap and a shift of the phase transitions towards lower temperatures due to the compositional disordering.