Heat capacity and thermodynamic properties of silver(I) selenide, oP-Ag 2Se from 300 to 406 K and of cI-Ag 2Se from 406 to 900 K: transitional behavior and formation properties

Abstract

Heat capacities of silver sulfide samples in the Ag 2Se 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 Ag 2Se to cubic Ag 2Se started at about 404.0 K in the two former samples on heating, but progressed remarkably slow with temperature for stoichiometric Ag 2Se. 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 Ag 2Se, 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; Δ trs H=6824±20 J mol −1 . Thermodynamic function values for Ag 2Se have been derived and are tabulated for selected temperatures. The composition range of cubic Ag 2Se phase widens with temperature on the Se-rich side and includes the composition Ag 2Se 1.01 at about 800 K.