The existence of the only ternary compound, Ag3AuSe2 (fischesserite), in the Ag-Au-Se system was confirmed 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

\begin{eqnarray*}&&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)\end{eqnarray*}

were studied by the electromotive force (EMF) technique in all solid-state galvanic cells with Ag4RbI5 as a solid electrolyte. The experiments were run in a dry argon gas flow at atmospheric pressure.

The following standard thermodynamic properties of the stable phases were determined at 298.15 K and 1 bar (105 Pa):

\begin{eqnarray*}&&{\Delta}_{f}\mathit{G}{^\circ}(Ag_{2}Se,\ low\ naumannite)\ =\ {-}49470\ ({\pm}130)\ J/mol,\\&&\mathit{S}{^\circ}(Ag_{2}Se,\ low\ naumannite)\ =\ 149.99\ ({\pm}0.56)\ J/(K{\cdot}mol);\\&&{\Delta}_{f}\mathit{G}{^\circ}(Ag_{2}Se,\ high\ naumannite)\ =\ {-}47430\ ({\pm}290)\ J/mol,\\&&\mathit{S}{^\circ}(Ag_{2}Se,\ high\ naumannite)\ =\ 169.01\ ({\pm}0.78)\ J/(K{\cdot}mol);\\&&{\Delta}_{f}\mathit{G}{^\circ}(Ag_{3}AuSe_{2},\ fischesserite)\ =\ {-}86450\ ({\pm}320)\ J/mol,\\&&\mathit{S}{^\circ}(Ag_{3}AuSe_{2},\ fischesserite)\ =\ 290.80\ ({\pm}1.26)\ J/(K{\cdot}mol);\\&&{\Delta}_{f}\mathit{G}{^\circ}({\beta}-AuSe)\ =\ {-}4110\ ({\pm}1300)\ J/mol;\\&&\mathit{S}{^\circ}({\beta}-AuSe)\ =\ 75.49\ ({\pm}3.55)\ J/(K{\cdot}mol).\end{eqnarray*}
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