The effect of temperature and pressure on the optical absorption edge in Cu6PS5X (X = Cl, Br, I) crystals

The absorption edge of monocrystalline Cu6PS5X (X = Cl, Br, I) superionic f erroelastics was studied in the 77-303 K temperature range and the mechanis ms of exciton-phonon interactions resulting in exponential absorption edges and Urbach tails were analysed. Various types of disordering, affecting th e optical absorption tails, were studied. The investigation of the influenc e of hydrostatic pressure (up to 400 MPa) on optical transitions of Cu6PS5X at 295 K revealed that the increase of pressure causes an increase of the optical pseudogap as well as the increase of the slope of the absorption ed ge indicating pronounced ordering in most disordered crystals. Data obtaine d from the pressure studies confirmed the predominant contribution of excit on-phonon interaction to the temperature shift of the Urbach absorption edg e. (C) 1999 Elsevier Science Ltd. All rights reserved.