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 °C. These temperatures vary systematically with composition, with highs at the pure end members, Ag₂S (178 °C) and Ag₂Se (134 °C), and a low in the compositional range of approximately Ag₂S_0.4Se_0.6to Ag₂S_0.3Se_0.7(70 °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 Ag₂S_0.4Se_0.6and the Ag₂Se-II-type, orthorhombic, extending from Ag₂Se to Ag₂S_0.3Se_0.7. 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.