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

The temperature behavior (4.2-320 K) of the fundamental absorption edge of Cu6P(S1-xSex)(5)I mixed crystals (x = 0.05, 0.1, 0.15, 0.2) was studied. Bo und and free direct excitons were detected at low temperatures in the diele ctric phase (T<T-s) for x = 0.05 and x = 0.1 crystals, at the absorption ed ge and in the photoluminescence spectra. The spectral and temperature behav ior of the long-wavelength side of the absorption edge in the superionic ph ase is described by the Urbach rule. Mechanisms of the exciton-phonon inter action, resulting in an exponential Urbach edge, were analyzed taking into account various types of disorder. Increasing of the selenium content resul ts in an increase in the energy width of the Urbach absorption edge, a decr ease of the optical bandgap and a shift of the phase transitions towards lo wer temperatures due to the compositional disordering. <(c)> 2001 Elsevier Science Ltd. All rights reserved.