Phase diagram of the Y–Y2Se3 system, enthalpies of phase transformations

Abstract

A phase diagram for the Y–Y2Se3 system has been constructed in which the YSe and Y2Se3 phases melt congruently. The daltonide type YSe phase (ST Y0,75Se, а=1.1393nm, melting point=2380K, Н=2200MPa) forms a double-sided solid solution from 49–50–53at% Se. In the 50–53at% Se range, the unit cell parameter increases to 1.1500nm, the microhardness increases to 4100MPа and electrical resistivity increases from 0.018 to 0.114Ωm. These changes are caused by the dominating influx of newly formed structural cationic vacancies arising from the selenium anions that are surplus for the 1:1 Y:Se stoichiometry. The full-valence Y2Se3 composition exists as a low-temperature modification of ε-Y2Se3 (SТ Sc2S3, a=1.145nm, b=0.818nm, c=2.438nm, melting point=1780K, ∆fusion enthalpy=4±0.4J/g) and transforms into a modification of ξ-Y2Se3 that does not undergo fixing by thermo-hardening. The eutectic melting point between the YSe and Y2Se3 phases is 1625±5K, with a eutectic composition that is assumed to be 57.5at% Se and have an enthalpy of fusion of 43±4.3J/g. The eutectic for the Y and YSe phases appears at a temperature of 1600K and 5at% Se.