Thermodynamic Properties of Saturated Solid Solutions of the Phases Ag 2 PbGeS 4 , Ag 0.5 Pb 1.75 GeS 4 and Ag 6.72 Pb 0.16 Ge 0.84 S 5.20 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 serpentine-like shape of the concentration variation of Gibbs partial energy of silver \(- \Delta \overline{G}_{\text{Ag}}^{ \circ }\) 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 \(- \Delta \overline{G}_{\text{Ag}}^{ \circ }\) 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|Ag|Ag2GeS3 glass|D|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 Ag6.48Pb0.19Ge0.81S5.05 from the PbS side). A significant excess of integral standard energy Gibbs \(\Delta_{\text{f}} G_{{298,\;{\text{Ag}}_{ 6. 4 8} {\text{Pb}}_{ 0. 1 9} {\text{Ge}}_{ 0. 8 1} {\text{S}}_{ 5. 0 5} }}^{\circ}\) = (−401.8 ± 1.5) kJ mol−1 over the values of \(\Delta_{\text{f}} G_{298,\;{\text{A}}{{\text{g}}_{\text{8}}}{\text{Ge}}{{\text{S}}_{\text{6}}}}^{\circ}\) = (−345.0 ± 2.2) kJ mol−1 and \(\Delta_{\text{f}} G_{{298,\;{\text{PbS}}}}^{\circ}\) = (−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 \(T\Delta_{\text{f}} S_{{298,\;{\text{Ag}}_{ 6. 4 8} {\text{Pb}}_{ 0. 1 9} {\text{Ge}}_{ 0. 8 1} {\text{S}}_{ 5. 0 5} }}^{\circ}\) into the value of \(\Delta_{\text{f}} G_{{298,\;{\text{Ag}}_{ 6. 4 8} {\text{Pb}}_{ 0. 1 9} {\text{Ge}}_{ 0. 8 1} {\text{S}}_{ 5. 0 5} }}^{\circ}\) exceeds 33%.