Thermodynamic Properties of Saturated Solid Solutions of the Phases Ag2PbGeS4, Ag0.5Pb1.75GeS4 and Ag6.72Pb0.16Ge0.84S5.20 of the Ag-Pb-Ge-S System Determined by EMF Method

Triangulation of the Ag-Pb-Ge-S system in the Ag8GeS6-Ge-GeS2-PbS part was performed by differential thermal analysis, XRD and EMF methods. The serpentinelike shape of the concentration variation of Gibbs partial energy of silver -Delta(G) over bar degrees(Ag) in the alloys of the Ag8GeS6-PbS system in the Ag8GeS6-(Ag8GeS6)(0.79)(PbS)(0.21) part at T = const (T = 518, 538, 556 K) is the consequence of the kinetically metastable state of the alloys of the limited solid solution range Ag8GeS6-(Ag8GeS6)(0.81)(PbS)(0.19). According to the principles of continuity and correspondence, the -Delta(G) over bar degrees(Ag) extremum of the alloy (Ag8GeS6)(0.84)(PbS)(0.16) determines the composition of the intermediate phase Ag6.72Pb0.16Ge0.84S5.20 of the equilibrium system Ag8GeS6-PbS. Potential-forming processes were performed in electrochemical cells (ECCs) of the type C vertical bar Ag vertical bar Ag2GeS3 glass vertical bar D vertical bar C (where C are current (graphite) electrode; Ag and D are the electrodes of ECCs; Ag2GeS3 glass is a membrane with ionic Ag+ conductivity; D are silver-containing single-, two- and four-phase alloys). Linear EMF dependences of cells in the 510-560 K range were used to calculate integral values of the thermodynamic functions of saturated solid solutions of the phases Ag2PbGeS4, Ag0.5Pb1.75GeS4, Ag6.72Pb0.16Ge0.84S5.20 (the composition of Ag6.48Pb0.19 Ge0.81S5.05 from the PbS side). A significant excess of integral standard energy Gibbs Delta(f)G(298)degrees(,) (Ag6.48Pb0.19Ge0.81S5.05) = (-401.8 +/- 1.5) kJ mol(-1) over the values of Delta(f)G(298)degrees(,) (Ag8GeS6) = (-345.0 +/- 2.2) kJ mol(-1) and Delta(f)G(298)degrees(, PbS) = (-96.994 +/- 0.005) kJ mol(-1) is considered as the confirmation of the existence of a quaternary phase in the equilibrium system Ag8GeS6-PbS. The contribution of the entropy factor T Delta S-f(298)degrees(, Ag6.48Pb0.19Ge0.81S5.05) into the value of Delta(f)G(298)degrees(,) (Ag6.48Pb0.19Ge0.81S5.05) exceeds 33%.