Heat capacity and thermodynamic properties of silver(I) selenide, oP-Ag2Se from 300 to 406 K and of cI-Ag2Se from 406 to 900 K: transitional behavior and formation properties

Heat capacities of silver sulfide samples in the Ag2Se composition region have been measured by adiabatic calorimetry from 298.15 to 900 K. In addition to a single-phase sample with stoichiometric composition, a two-phase sample with 1 mol% silver excess, and one with similar selenium excess were studied. The structural transition from orthorhombic Ag2Se to cubic Ag2Se started at about 404.0 K in the two former samples on heating, but progressed remarkably slow with temperature for stoichiometric Ag2Se. The transition was 90% complete at 406.45 K, compared to 96% completion already at 405.59 K for the sample with silver excess. For the selenium-rich sample the transition was 93% complete at 405.8 K, or about 0.6 K below similar completion for the practically stoichiometric Ag2Se, which thus has the highest transition temperature. A phase diagram complying with the observations is drawn. In contrast to an earlier finding the transitional enthalpy does not vary appreciably with composition; ΔtrsH=6824±20 J mol−1. Thermodynamic function values for Ag2Se have been derived and are tabulated for selected temperatures. The composition range of cubic Ag2Se phase widens with temperature on the Se-rich side and includes the composition Ag2Se1.01 at about 800 K.