Calorimetric analysis of the system Ag₂S–Ag₂Se between 25 and 250 °C

Differential scanning calorimetry (DSC) demonstrates that compounds in the pseudobinary system Ag2S-Ag2Se undergo rapid, reversible solid-state phase changes at temperatures between approximately 70 and 178-degrees-C. These temperatures vary systematically with composition, with highs at the pure end members, Ag2S (178-degrees-C) and Ag2Se (134-degrees-C), and a low in the compositional range of approximately Ag2S0.4Se0.6 to Ag2S0.3Se0.7 (70-degrees-C). These data are consistent with the presence of two solid solutions in this system at ambient conditions: the Ag2S-III-type, monoclinic, ranging to approximately Ag2S0.4Se0.6 and the Ag2Se-II-type, orthorhombic, extending from Ag2Se to Ag2S0.3Se0.7. Entropies of transition of 6 to 9 e.u. characterize compositions within the Ag2S-III-type solid solution, whereas values of 13 to 16 e.u. mark members of the Ag2Se-II-type solid solution. The high-temperature allotrope is presumably a continuous solid solution between Ag2S-II and Ag2Se-I, which have similar body-centered cubic structures.