Effect of fluctuations on electron and phonon processes and thermodynamic parameters of Ag2Te and Ag2Se in the region of phase transition

Temperature dependences of electrical conductivity σ, thermoelectric power α, results of differential thermal analysis ΔT _y , thermal conductivity χ, temperature conductivity κ, and heat capacity C _p were studied in Ag₂Te and Ag₂Se semiconductors in the region of the phase transition. Two extrema are observed in the temperature dependence χ(T): a maximum in the region of the α′ → β′ transition and a minimum in the region of the β′ → β transition; these extrema are caused by the similar dependence C _p (T). It is shown that the α → α′ and β′ → β transitions are displacement transitions, while the α′ → β′ transition is of reconstruction type. It is established that the disorder parameter η in silver chalcogenides is highly smeared in the region of the phase transition; therefore, disordering of phases at the point of the phase transition is incomplete: 73, 62, and 48% in Ag₂Te, Ag₂Se, and Ag₂S, respectively. The minimum volumes V _ph for new phases are calculated; it is shown that the value of V _ph in displacement transitions is larger than in the reconstruction-type transitions.