Optical, electron microprobe, and x-ray diffraction analysis of 88 samples of various compositions between Ag2S and Ag2Se synthesized at high temperature in sealed quartz tubing indicates the presence of two solid-solution series in this system at ambient (room) conditions. One series extends from Ag2S to approximately Ag2S0.4Se0.7 and has the Ag2S-III-type structure (monoclinic). The second series ranges from Ag2S0.3Se0.7 to Ag2Se and is characterized by the Ag2Se-II-type structure (orthorhombic). Members of both series, in appropriate proportions, characterize the apparent compositional gap between the two solid solutions. Gradual shifts in the locations of the x-ray diffraction peaks along the compositional gradient of each solid solution revealed an expansion of the d-spacing as the larger Se ion was substituted for S in the Ag2S-III-type structure and a contraction as S was substituted for Se in the Ag2Se-II-type structure. The reported discrete phase, Ag4SSe (aguilarite, orthorhombic), appears to be simply a member of the monoclinic Ag2S-III-type solid solution.
Differential scanning calorimetry (DSC) demonstrates that compounds in the pseudobinary system Ag 2 S-Ag 2 Se 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, Ag 2 S (178 °C) and Ag 2 Se (134 °C), and a low in the compositional range of approximately Ag 2 S 0 .4Se 0 .6 to Ag 2 S 0 .3Se 0 .7 (70 °C). These data are consistent with the presence of two solid solutions in this system at ambient conditions: the Ag 2 S-III-type, monoclinic, ranging to approximately Ag 2 So.4Se o . 6 and the Ag 2 Se-II-type, orthorhombic, extending from Ag 2 Se to Ag 2 S 0 .3Se 0 .7. Entropies of transition of 6 to 9 e.u. characterize compositions within the Ag 2 S-III-type solid solution, whereas values of 13 to 16 e.u. mark members of the Ag 2 Se-II-type solid solution. The high-temperature allotrope is presumably a continuous solid solution between Ag 2 S-II and Ag 2 Se-I, which have similar body-centered cubic structures.
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