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

Temperature behaviour (77-320 K) of the total electrical conductivity and the fundamental absorption edge of Cu6P(SexS1-x)5I mixed crystals is studied for compositions x = 1.0, 0.9, 0.8. In the range of the superionic phase-transition sharp stepwise changes of the electric conductivity st and optical pseudo-gap as well as a temperature hysteresis in st are detected, thus supporting the evidence that the discussed phase transitions are of the first-order. The spectral and temperature behaviour of the absorption edge in both phases is described by the Urbach rule, however, the convergence point coordinates are different for the dielectric and superionic phases. Various types of disorder, affecting the absorption processes in the long-wavelength side of the absorption edge, are considered. The higher sulfur content results in the increase of width of the Urbach absorption edge, , and optical pseudo-gap, , along with the phase transitions smearing, due to compositional disordering.

Fast-ion conductors; phase transition; electric conductivity; absorption edge; Urbach tail; structural disorder; compositional disorder

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