Stable and metastable phase equilibria in the GeSe2-Se system

Stable and metastable phase equilibria in the binary system GeSe2-Se have been investigated by means of optical and electron microscopy, electron probe microanalysis (EPMA), x-ray diffraction (XRD), thermal analysis, and adiabatic calorimetry. A revised equilibrium phase diagram is presented. A not previously observed λ-type phase transition was observed near ∼420 K (147 °C) for GeSe2. The intermediate phase, φ, which earlier had been considered as metastable, is shown to be stable through extensive annealing experiments and by adiabatic calorimetry. The φ phase, with composition near Ge3Se7, melts incongruently at 658±5 K (385±5 °C) to GeSe2 and a liquid with 85±2 at.% Se. The liquidus of GeSe2 and φ was determined by EPMA of annealed-and-quenched samples. Both a stable (φ-Se-liquid) and a metastable (GeSe2-Se-liquid) eutectic point were observed. The stable one was observed at 480±2 K (207±2 °C) and 92.5±1.0 at.% Se, whereas the temperature of the metastable eutectic (GeSe2-Se-liquid) was observed to be ∼10 K lower. Metastable phase equilibria, obtained on thermal evolution of glasses with different compositions, are described and discussed.