Specific heat and thermal conductivity of ionic conductors and chalcogenide glasses at low temperatures

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Abstract
Measurements of specific heat in the temperature range 1–40 K for four chalcogen amorphous alloys (Se1-x-Tex)1-ySby and of thermal conductivity for three samples of superionic glasses (AgI)x (Ag2O2B2O3)1-x between 1 and 10 K are reported. The anomalous measured specific heat is interpreted in terms of phonon localization. The values obtained for the parameter L characterizing the length of this localization confirm the existing hypotheses on the structural role of Te and Sb atoms in a Se matrix. The thermal conductivity results are explained in terms of phonon scattering by two-level systems, typical of the glassy state, dominant at low temperatures, and by fluctuations in bond length, characterized by a fractional means square variation different from zero, at higher temperatures. The best fit of the experimental curves gives the value of the mean-square variation in bond length and gives the value of the state density of the two-level system and the coupling constant with the phonons.