Phase equilibria in Tl2Se–In2Se3–GeSe2 system at 520 K

Abstract

Isothermal section of the quasi-ternary Tl 2 Se–In 2 Se 3 –GeSe 2 system at 520 K has been studied by means of X-ray phase analysis. 52 samples have been prepared from high-purity elements: thallium, germanium, indium and selenium, which were co-melted in evacuated quartz-glass ampoules. The synthesis was performed in a shaft-type furnace. The ampoules were heated up to 950 K at a rate of 30 K/h, with 5 h exposure at the maximum temperature. Homogenization annealing of samples was conducted at 520 K for 240 h. After annealing, the ampoules with samples were quenched down to room temperature in air. X-ray diffraction patterns of the samples were recorded with DRON 4-13 diffractometer, Cu K α-radiation, 2θ range 10–80°, scan step 0.05°, exposure time 5 s in each point. Five ternary compounds – TlInSe 2 , TlIn 5 Se 8 , Tl 4 GeSe 4 , Tl 2 GeSe 3 , and Tl 2 Ge 2 Se 5 – have been confirmed to occur in the limiting quasi-binary systems Tl 2 Se–In 2 Se 3 and Tl 2 Se–GeSe 2 at 520 K. No compounds have been found in In 2 Se 3 –GeSe 2 system, which is consistent with the literature data. On the TlInSe 2 –GeSe 2 cross-section, the occurrence of two quaternary compounds TlInGeSe 4 and TlInGe 2 Se 6 has been confirmed. No new phases were formed at the annealing temperature being stable to room temperature. Positions of 10 single-phase, 19 two-phase and 10 three-phase fields have been identified. The isothermal section is characterized by low solid solubility based on binary (Tl 2 Se, GeSe 2 , In 2 Se 3 ), ternary and quaternary compounds. The homogeneity region of solid solution TlIn 1- x Ge x Se 2 has the largest extent (up to 25 mol. % GeSe 2 ) along the quasi-binary section TlInSе 2 –GeSе 2 , which is consistent with the results of earlier studies. Keywords: quasi-ternary system, isothermal section, phase equilibria, crystal structure.