Thermodynamic properties of the AgSnTe2 compound

Abstract

The phase equilibria of the Ag–Sn–Te system in the Ag 2 Te–SnTe–Te part were investigated by the X-ray and electromotive force method. The spatial location of the established phase regions Ag 2 Te–SnTe–AgSnTe 2 and AgSnTe 2 –SnTe–Te relative to composition of Ag were used to express the chemical reactions of decomposition and synthesis of the AgSnTe 2 compound with the participation Ag, Ag 2 Te, SnTe and Ag, SnTe, Te, respectively. The potential-forming reactions were performed by applying electrochemical cells (−) C | Ag | Ag 2 GeS 3 glass | D | C (+), where C is graphite, Ag is a reference electrode, Ag 2 GeS 3 -glass is the fast purely Ag + ions conducting electrolyte, and D is an three-element equilibrium mixture of phases (the right electrode). Components of the electrochemical cells in powder form were pressed at 10 8 Pa through a 2 mm diameter hole arranged in the fluoroplast matrix up to density ρ = (0.93 ± 0.02)× ρ 0 , where ρ 0 is the experimentally determined density of cast samples. Five-fold thermal cycling of the cells in the temperature range of 400–500 K was performed to eliminate possible defects due to plastic deformation during sample pressing. The linear dependences of the EMF of cells with positive electrodes of the Ag 2 Te–SnTe–AgSnTe 2 and SnTe–Te–AgSnTe 2 phase regions were obtained in the temperature range of 468–526 K. EMF data were used to calculate the Gibbs energy of formation of the AgSnTe 2 compound in the Ag 2 Te–SnTe–AgSnTe 2 and AgSnTe 2 –SnTe–Te phase regions, respectively: /(kJ·mol − 1 ) = −(74.41±2.89) − (21.71±0.82)·10 − 3 Т /K; /(kJ·mol − 1 ) = −(77.53±2.97) − (20.11±0.76)·10 − 3 Т /K. The calculated Gibbs energy of the ternary compound in both phase regions are consistent, which indicates that region of the solid solution based on AgSnTe 2 compound is narrow. Determined in this work standard thermodynamic functions of the ternary compound can be used for modeling of the T–x diagrams of the Ag–Sn–Te system. Keywords: Silver–based phases, Phase equilibria, Thermodynamic properties, EMF method, Gibbs energy.