THERMODYNAMIC PROPERTIES OF SOLID SOLUTIONS IN THE Cu8GeS6-Ag8GeS6 SYSTEM
Abstract
The system Cu8GeS6-Ag8GeS6 was studied by electromotive force measurements of the
(−)Cu(sol.)|Cu4RbCl3I2(sol.)|(Cu in alloys)(sol.)(+)(1)(1)(−)Cu(sol.)|Cu4RbCl3I2(sol.)|(Cu in alloys)(sol.)(+)concentration chains within 300÷380 К temperature interval. In these chains, solid-state superionic conductor Cu4RbCl3I2 was used as the electrolyte. This compound possesses high ionic conductivity (0.5 Om-1·sm-1) on Cu+ cations at a room temperature. The metallic copper was used as a left (negative) electrode and equilibrium alloys of the studied system were served as right (positive) electrodes. The formation of wide areas of solid solutions based on Cu8GeS6 (α-phase, 60 mol.%) and Ag8GeS6 (β-phase, 23 mol.%) starting compounds is defined. Based on EMF measurements, the linear equations of temperature dependences EMF were obtained and presented in the following form
E=a+bT±t[S2En+S2b(Ti−T¯¯¯¯)2]1/2(2)(2)E=a+bT±t[SE2n+Sb2(Ti−T¯)2]1/2where n is the number of pairs of E and T values; S2E and S2b are the error variances of the EMF readings and b coefficient, respectively.
The partial molar thermodynamic functions of copper in alloys were calculated based on the results of the EMF measurements.
Analysis of the concentration dependences of the partial molar functions of copper showed that the within homogeneity areas of α and β-phase with a decrease of the copper content the partial entropy increases monotonically, while the partial enthalpy and Gibbs free energy decrease. At the interfaces of α- and β-phases with two-phase α + β field, there are abrupt changes in the partial entropy and enthalpy. All partial molar functions of copper are constant in the α+β two-phase area, which is related with the constancy of compositions of coexisting phases.
Keywords: Cu8GeS6 - Ag8GeS6 system, solid solutions, EMF method, Cu4RbCl3I2 solid electrolyte, thermodynamic properties
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References
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