The Ag-Au-S system

Determination of phase relations in the Ag-Au-S system between 100° and 850°C by silica tube, differential thermal analysis, and high-temperature X-ray experiments reveals that the condensed system consists of three basic parts: the Ag-Au complete solid solution, the Ag ₂S-Au ₂S partial solid solution, and sulfur. At low temperatures, two ordered silver-gold sulfides exist, Ag ₃AuS ₂ and AgAuS; they invert at 185° and 310°C, respectively, to a simple, cubic solid solution. Above 310°C a complete solid solution exists from Ag ₂S to approximately Ag _0.9Au _1.1S. This solid solution has a body- centered cubic structure for silver-rich compositions, which changes via a second-order transition to a simple, cubic structure between Ag ₂S and Ag _1.5Au _0.5S. The silver-rich end of the solid solution inverts to a face-centered modification at high temperatures. The sulfides melt in the interval 680° to 838°C with a minimum in the liquidus at Ag _1.3Au _0.7S. Enthalpies of transition for the Ag ₂S III -II, Ag ₃AuS ₂ low-high, and AgAuS low-high transitions were measured by differential scanning calorimetry and found to be, respectively: 950; 2,270(min)-2,760(max); and l,570(min)-l,920(max) calories/mole. Within experimental error, the sulfide solid can be modeled as a regular solution with components AgS _0.5 and AuS _0.5 and with Wg = -1,000 calories. Estimated temperature- dependent standard free energies of formation are (T = K and units of calories): -43,880 + 20.8T (acanthite, Ag ₂S III), - 40,410 + 20.8T (uytenbogaardtite, low Ag ₃AuS ₂), -37,910+ 15.3T (high Ag ₃AuS ₂), -36,740 + 23.4T (low AgAuS), -33,340 + 17.6T (high AgAuS), and -22,700 + 29.6T (Au ₂S).