Electric conductivity and optical absorption edge of Cu6P(SexS1-x)(5)I fast-ion conductors in the selenium-rich region

Temperature behavior (77-320 K) of the total electrical conductivity and th e fundamental absorption edge of Cu6P(SexS1-x)(5)I mixed crystals is studie d for compositions x = 1.0, 0.9, 0.8. In the range of the superionic phase- transition sharp stepwise changes of the electric conductivity sigma (t) an d optical pseudo-gap E-g* as well as a temperature hysteresis in sigma (t) are detected, thus supporting the evidence that the discussed phase transit ions are of the first-order. The spectral and temperature behavior of the a bsorption edge in both phases is described by the Urbach rule, however, the convergence point coordinates are different for the dielectric and superio nic phases. Various types of disorder, affecting the absorption processes i n the long-wavelength side of the absorption edge, are considered. The high er sulfur content results in the increase of width of the Urbach absorption edge, w, and optical pseudo-gap, E-g*, along with the phase transitions sm earing, due to compositional disordering. (C) 2001 Elsevier Science Ltd. Al l rights reserved.