CALORIMETRIC ANALYSIS OF THE SYSTEM AG2S-AG2SE BETWEEN 25-DEGREES-C AND 250-DEGREES-C

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