Thermodynamic study of the systems PrSe2.00–PrSe1.50 and GdSe1.875–GdSe1.50

A detailed thermodynamic study of the systems PrSe2.00–PrSe1.50 and GdSe1.875–GdSe1.50 was performed using a static method membrane-gauge manometer within the temperature range 675–1296 K. High-quality crystals of PrSe2.00 and GdSe1.875 were investigated. The p–T–x diagrams obtained in this study show that the regions between the boundary compositions PrSe2.00 and PrSe1.80, GdSe1.86 and GdSe1.77 which were previously described as non-stoichiometric, consist of discrete stoichiometric phases such as PrSe1.90, PrSe1.85, PrSe1.80 and GdSe1.85, GdSe1.80, GdSe1.71, respectively. The compositions of the polyselenides were evaluated from the static data to the accuracy of ±0.02 formula units. From the enthalpy and the entropy of thermal dissociation, the standard heats of formation and the absolute entropies were estimated for each polyselenide.