Thermodynamic stability of Ag₂Se from 350 to 500 K by a solid state galvanic cell

The numerical values on the standard thermodynamic functions of Ag ₂Se (naumannite) were determined by the electromotive force (EMF) method in a solid-state galvanic cell with superionic conductor RbAg ₄I₅ as the solid electrolyte. Ag₂Se was synthesized from pure elements in evacuated quartz glass ampoules and examined to be homogenous by SEM and EDS. According to the experimental data on the EMF versus temperature, the analytical equations were obtained for the polymorphic forms of Ag₂Se. The temperature-dependent relationships of the Gibbs energy of formation of Ag₂Se in its polymorphic forms and the standard thermodynamic functions of compounds within the temperature range of 350-500 K were also evaluated. The temperature of phase transformation from α-Ag₂Se to β-Ag₂Se is determined experimentally to be 407.7 K and the enthalpy of phase transformation is 6.06 kJ mol ^{- 1}.