(711d) Phase Diagram Determination of Thermoelectric Ag-Pb-Sn-Te Quaternary System

Ag-Pb-Sn-Te is a quaternary system of high thermoelectric application importance. SnTe, PbTe, Ag₂Te are all promising thermoelectric compounds. It has been reported that their thermoelectric properties could be further enhanced by doping with various elements, such as Ag, Pb, Sn. Phase diagrams provide fundamental materials information and are important for materials design, processing route determination and illustration of formation of phases and microstructures. This study determines phase diagram of the Ag-Pb-Sn-Te quaternary system. Phase diagrams of its constituent binary systems, Ag-Pb, Ag-Sn, Ag-Te, Pb-Sn, Pb-Te and Sn-Te, are available in the literatures. Phase diagrams of two constituent ternary systems, Ag-Pb-Te and Pb-Sn-Te, are also available, but those are lacking of other two systems, Ag-Pb-Sn and Ag-Sn-Te. Phase diagrams at 350^oC and 500^oC of these two ternary systems, Ag-Pb-Sn and Ag-Sn-Te, are experimentally determined. Ag-Pb-Sn-Te quaternary alloys of selected compositions are prepared and their equilibrated phases are determined. No ternary or quaternary compounds are observed. The stable binary compounds at 350^oC and 500^oC are ζ-Ag₄Sn, ε-Ag₃Sn (not stable at 500℃), Ag₂Te, Ag₅Te₃ (not stable at 500℃), PbTe and SnTe. Pb(Se,Te) and (Pb,Sn)Te are continuous solid solutions. There are two tie-triangles, Ag+ε-Ag₃Sn +Liquid and ε-Ag₃Sn+ζ-Ag₄Sn+Liquid, in the Ag-Pb-Sn ternary system at 350℃, and there is only one tie-triangle Ag+ζ-Ag₄Sn+Liquid, at 500℃. There are five tie-triangles in the Ag-Sn-Te system at 350℃, Liquid+ε-Ag₃Sn+SnTe, ε-Ag₃Sn+ζ-Ag₄Sn+SnTe, ζ-Ag₄Sn+Ag₂Te+SnTe, ζ-Ag₄Sn+Ag+Ag₂Te, and SnTe+Ag₂Te+Te. The 500℃ isothermal section is similar, but without ε-Ag₃Sn phase and Te is molten, and has four tie-triangles, Liquid+ζ-Ag₄Sn+SnTe, ζ-Ag₄Sn+Ag₂Te+SnTe, ζ-Ag₄Sn+Ag+Ag₂Te, and SnTe+Ag₂Te +Liquid. The Ag-Pb-Se-Te isoplethal sections at 50.0 at%Te and 33.3 at%Te at 350^oC and 500^oC are then determined. The experimental results are used for the Calphad-type modeling of the Ag-Pb-Se-Te system for phase diagram calculation.