New insights into phase equilibria of the SnTe-Bi2Te3 pseudo-binary system: Synthesis and crystal structure of new tetradymite-type compound Sn3Bi2Te6

The phase relationship in the SnTe-Bi2Te3 pseudo-binary system has been reinvestigated experimentally by means of powder X-ray diffraction (XRD), differential thermal analysis (DTA), and scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) techniques to obtain a better description of the phase diagram. The new version of the phase diagram differs from previous versions. The formation of a total of 5 tetradymite-type ternary compounds has been experimentally confirmed, including 4 compounds mentioned in the literature and, for the first time, a new compound Sn3Bi2Te6. The newly revealed phase was determined to be a rhombohedral structure (space group R-3m, No. 164, lattice parameters: a = b = 4.40935 angstrom, c = 63.8370 angstrom in hexagonal representation) based on the powder XRD data and melts with solid-phase reaction, while other reconfirmed ternary compounds have peritectic melting character. Furthermore, the solid solubility of starting binary compounds SnTe and Bi2Te3 is established to be -12 and 9 mol % at 450 degrees C, respectively. A eutectic temperature of 575 degrees C was observed at a composition of -85 mol% Bi2Te3. The current updated version of the phase diagram can provide the essential information for alloying strategies and guide the designing of the Sn-Bi-Te-based high-performance functional materials.