The existence of the only ternary compound, Ag 3 AuSe 2 (Þ schesserite), in the Ag-Au-Se system was conÞ rmed by solid-state annealing method. The selenium analog of petrovskaite (AgAuS) was not observed under the experimental conditions (in the temperature range of 350-773 K and own vapor pressure).
The solid-state reactions 2Ag(cr) + Se(s) = Ag 2 Se(cr) Ag(cr) + Ag 3 AuSe 2 (cr) = 2Ag 2 Se(cr) + Au(cr) 3Ag(cr) + 2AuSe(s) = Ag 3 AuSe 2 (cr) + Au(cr)
were studied by the electromotive force (EMF) technique in all solid-state galvanic cells with Ag 4 RbI 5 as a solid electrolyte. The experiments were run in a dry argon gas ß ow at atmospheric pressure.
The following standard thermodynamic properties of the stable phases were determined at 298.15 K and 1 bar (10 5 Pa):
S°(Ag 2 Se, low naumannite) = 149.99 (±0.56) J/(K•mol); ∆ f G°(Ag 2 Se, high naumannite) = -47430 (±290) J/mol, S°(Ag 2 Se, high naumannite) = 169.01 (±0.78) J/(K•mol); ∆ f G°(Ag 3 AuSe 2 , Þ schesserite) = -86450 (±320) J/mol, S°(Ag 3 AuSe 2 , Þ schesserite) = 290.80 (±1.26) J/(K•mol); ∆ f G°(β-AuSe) = -4110 (±1300) J/mol; S°(β-AuSe) = 75.49 (±3.55) J/(K•mol).
The phase transition point for low naumannite-high naumannite was determined at T trs = 405.4 K, with the enthalpy of transition of ∆ trs H = -7713 (±550) J/mol.