Thermal Analysis of the YSe-Y2Se3 system

Abstract

The solidus-liquidtis area of the phase diagram of the YSe-Y2Se3 system is constructed by the visual-polythermal (VPTA) and simultaneous-thermal (STA) analyses. While finding the Y:,Ses.x compound in the polycrystalline state, the endothermic effect isfirst registered at t = 1,325 ± 5 °C; AH = 18 ±2 kJ/mol (43 J/g). The effect is identified as the polymorphic transition from the Y2Se3.x (ST Sc2S) orthorhombic structure into the high-temperature modification of </-Y2Se3, most probably of a cubic structure. The transition is reversible, it is reproduced when being cooled. The eutectoid transformation temperature in the Y2Se.-based solid solution is equal to 1,275 ± 5 °C. The heat effect of eutectic melting is registered in the thermal analysis of the Y2Se3.xcompound. The total melting enthalpy of all the Y,Se3.x crystals is approximately 70 ± 15 kJ/mol (169 J/g). According to the VPTA, the eutectic temperature between the YSe and Y2Se3 phases is 1,380 ± 15 °C; according to the STA, it is 1,350 ± 7 °C. The eutectic composition is assumed to be 57.5 at. % Se; according to the STA, the melting enthalpy of the eutectics is 43.2 ±5 J/g. The melting temperature of YSe is 2,110 ± 35 °C.