PHASE DIAGRAMS IN THE DEVELOPMENT OF THALLIUM-REE TELLURIDES WITH Tl5Te3 STRUCTURE AND MULTICOMPONENT PHASES BASED ON THEM OVERVIEW

Abstract

Ternary structural analogues of Tl5Te3 are of considerable scientifi c and practical interest as promising functional materials with thermoelectric, optical, and magnetic properties, as well as topological insulators properties and superconductivity. The study included the systematization of the literature about the ternary lanthanides containing structural analogues of this compound and multicomponent phases based on them. Particular attention is paid to the works dedicated to the study of the phase equilibria in ternary and more complex systems forming phases with the Tl5Te3 structure. In a number of works, some polythermal and isothermal sections of the phase diagram, as well as projections of the liquidus and solidus surfaces, were constructed for the Tl2Te–Tl5Te3–Tl9LnTe6 concentration areas of the ternary systems Tl-Ln-Te (Ln-Nd, Gd, Sm, Tb, Tm). It was established that all types of Tl9LnTe6 ternary compounds melt with decom-position by a peritectic reaction. They are phases of variable composition (δ-phase) and their homogeneity region occupies more than 90% of the Tl2Te–Tl5Te3–Tl9LnTe6 concentration triangle. The phase equilibria along the Tl2Te–Tl9LnTe6–Tl9BVTe6 (I) Tl5Te3–Tl9LnTe6–Tl4AIVTe3(Tl9BVTe6) (II) and Tl9LnTe6–Tl4AIVTe3–Tl9BVTe6 (III)(AIV-Sn, Pb; BV-Sb, Bi) concentration planes of the corresponding quaternary and more complex systems was also studied in detail. It was established that sections (I) are characterised by the formation of wide areas, whereas sections (II) and (III) are unlimited solid solutions with a Tl5Te3 structure. The features of the crystal structure, the thermodynamic and some physical properties of these compounds, and the phases of variable composition, were considered. The constructed T-x-y diagrams and their various isothermal sections can be used to select alloy compositions used to grow single crystals of the δ-solid solutions with a given composition by the method of directional crystallization.

Keywords

• thallium chalcogenides
• structural analogues of Tl5Te3
• thallium-REE tellurides
• crystal structure
• phase diagrams
• solid solutions
• thermoelectric materials.