Unraveling the Tl–Pb–Te System: Structural Identity of Tl2Te and Tl5Te3

The present study investigates phase equilibria in the Tl–Pb–Te ternary system, which has been identified as a promising basis for the development of functional thermoelectric and topological materials. By applying differential thermal analyses, XRD, XPS and microstructural studies, it has been possible to resolve the conflicting issues regarding the distinctiveness of the binary phases Tl2Te and Tl5Te3. In a study of high-quality single crystals grown by directed crystallization, the existence of Tl2Te as a distinct compound with a monoclinic lattice (space group C2/c (№15), a = 15.610 Å, b = 8.914 Å, c = 31.069 Å, β = 100.5°) was confirmed, in contrast to the tetragonal Tl5Te3 (space group I4/mcm (№140), a = 8.931 Å, c = 12.595 Å). A triangulation scheme for the Tl–Pb–Te system was constructed on the basis of an XRD analysis of samples that were annealed at 473 K. The existence of eight quasi-binary cross-sections was experimentally confirmed. The study of quasi-binary and partially quasi-binary phases allowed us to determine the nature of phase transitions, homogeneity boundaries, and non-variant equilibrium temperatures. It has been demonstrated that the ternary compound Tl4PbTe3 forms broad solid-solution regions with Tl5Te3 (over 85 mol%) and Tl2Te (over 80 mol%). The constructed phase diagrams provide the physicochemical basis for the directed growth of high-quality single crystals with reproducible thermoelectric properties.