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Isoabsorption and spectrometric studies of optical absorption edge in Cu_6AsS_5I superionic crystal

Cu_6AsS_5I single crystals were grown using chemical vapour transport method. Two low-temperature phase transitions (PT) are observed from isoabsorption studies: a first-order PT at ТІ=153±1 K and a second-order PT in the temperature interval ТІI=260–280 K. At low temperatures and high absorption levels an excitonic absorption band was revealed in the range of direct optical transitions. At Т>ТІ, the absorption edge has an exponential form and a characteristic Urbach bundle is observed. The influence of the cationic P->As substitution on the parameters of the Urbach absorption edge, parameters of exciton-phonon interaction, and phase transitions temperatures is studied.

superionic conductors; phase transitions; optical properties

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