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.) | (Cuin alloys) (sol.) (+) (1) concentration chains within 300÷380 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+ cationsat a room temperature. The metallic copper was used as a left (negative) electrodeand equilibrium alloys of the studied system were served as right (positive) electrodes. The formation of wide areas of solid solutions based on Cu8GeS6 (a-phase, 60 mol.%) and Ag8GeS6 (b-phase, 23 mol.%) starting compounds is de ned. Based on EMF measurements, the linear equations of temperature dependences EMF wereobtained and presented in the following form E a bT t SnE S T Tb i = + ± + -ÈΠ͢˚ ˙22 21 2( )/ where n is the number of pairs of E and T values; SE 2 and Sb 2 are the error variances of the EMF readings and b coef cient, 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 and -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 - and -phases with two-phase a + b eld, there are abrupt changes in the partial entropy and enthalpy. All partial molar functions of copper are constant in the a+b 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.