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

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, a=1.1393 nm, melting point=2380 K, H=2200 MPa) forms a double-sided solid solution from 49-50-53 at% Se. In the 50-53 at% Se range, the unit cell parameter increases to 1.1500 nm, the microhardness increases to 4100 MPa and electrical resistivity increases from 0.018 to 0.114 Omega m. 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 epsilon-Y2Se3 (ST Sc2S3, a=1.145 nm, b=0.818 nm, c=2.438 nm, melting point = 1780 K, Delta fusion enthalpy=4 +/- 0.4 J/g) and transforms into a modification of zeta-Y2Se3 that does not undergo fixing by thermo-hardening. The eutectic melting point between the YSe and Y2Se3 phases is 1625 +/- 5 K, with a eutectic composition that is assumed to be 57.5 at% Se and have an enthalpy of fusion of 43 +/- 4.3 J/g. The eutectic for the Y and YSe phases appears at a temperature of 1600 K and 5 at% Se. (C) 2015 Elsevier Inc. All rights reserved.