Determination of phase relations in the Ag-Au-S system between 100 degrees and 850 degrees 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 2 S-Au 2 S partial solid solution, and sulfur. At low temperatures, two ordered silver-gold sulfides exist, Ag 3 AuS 2 and AgAuS; they invert at 185 degrees and 310 degrees C, respectively, to a simple, cubic solid solution. Above 310 degrees C a complete solid solution exists from Ag 2 S to approximately Ag (sub 0.9) Au (sub 1.1) S. 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 2 S and Ag (sub 1.5) Au (sub 0.5) S. The silver-rich end of the solid solution inverts to a face-centered modification at high temperatures. The sulfides melt in the interval 680 degrees to 838 degrees C with a minimum in the liquidus at Ag (sub 1.3) Au (sub 0.7) S.Enthalpies of transition for the Ag 2 S III-II, Ag 3 AuS 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 1,570(min)-1,920(max) calories/mole. Within experimental error, the sulfide solid can be modeled as a regular solution with components AgS (sub 0.5) and AuS (sub 0.5) and with W G [asymp] - 1,000 calories. Estimated temperature-dependent standard free energies of formation are (T = K and units of calories): -43,880 + 20.8T (acanthite, Ag 2 S III), -40,410 + 20.8T (uytenbogaardtite, low Ag 3 AuS 2 ), -37,910 + 15.3T (high Ag 3 AuS 2 ), -36,740 + 23.4T (low AgAuS), -33,340 + 17.6T (high AgAuS), and -22,700+ 29.6T (Au 2 S).

The Ag-Au-S system

Phase transitions in silver gold chalcogenides

Das System Ag2SeAg2S wurde mittels differential-thermischer, Rontgen-Phasen- und Mikrostrukturanalyse sowie Bestimmung der Dichte und der Mikroharte untersucht. Es wurden Gleichgewichtsproben verwendet, die bei Temperung bei 200, 400 und 500° erhalten wurden. Das System Ag2SeAg2S ist ein polythermischer (nicht quasibinarer) Schnitt des dreifachen Diagramms AgSeS. Es ist durch das Vorhandensein von Mischkristallen im ganzen Konzentrationsintervall gekennzeichnet mit Ausnahme der Zusammensetzung die 50% Ag2S entspricht. Ein peritektischer Zerfall wird bei 835° und einer Zusammensetzung von 60 Mol-% Ag2S beobachtet.

Untersuchung des Systems Ag2Se‐Ag2S

The phase diagram of the system Ag2−xAuxS(0 ⩽ x ⩽ 1) was studied by DTA, DSC, and high-temperature X-ray measurements. Three ordered compounds exist at lower temperatures: β-Ag2S, β-Ag3AuS2 and β-AgAuS. At higher temperatures a solid solution with α-Ag2S structure (body-centered cubic, b.c.c.) exists for 0 ⩽ x ⩽ 0.29 (at a temperature of 250 °C). For x > 0.29 (at 250 °C) the solid solution is simple cubic (s.c.); no miscibility gap between these two phases has been observed. Landau's theory of second-order transitions was applied to show that the space groups of the b.c.c. and the s.c. forms (Im3m and Pn3m, respectively) allow a continuous transition. A structure model analogous to Rahlfs' statistical model of α-Ag2S has been applied to the body-centered and simple cubic solid solutions. The intensities of the X-ray reflections show that silver and gold occupy different positions, the gold atoms occupy, statistically, the positions 1 4 1 4 1 4 , 3 4 3 4 1 4 , 1 4 3 4 3 4 , 3 4 1 4 3 4 in the s.c. form; in the b.c.c. form the positions 3 4 3 4 3 4 , 3 4 1 4 1 4 , 1 4 3 4 1 4 , 1 4 1 4 3 4 are also occupied. Gold is in linear coordination by sulfur. Silver occupies the same positions as found for α-Ag2S. Both cubic phases are highly disordered; during the transition from b.c.c. to s.c. the disorder decreases.

Order-disorder transitions in system ag2-xauxs (0 less-than-or-equal-to x less-than-or-equal-to 1)

The system Ag2−xAuxSe (0 ⩽ x ⩽ 0.5) is pseudo-binary. Two ordered compounds exist at lower temperatures, viz., β-Ag2Se and β-Ag3AuSe2. At higher temperatures a solid solution with the α-Ag2Se (body-centered cubic) structure exists over the whole composition range. The electronic conductivity of the b.c.c. solid solution decreases rapidly with increasing gold content; for x = 0.4 and 0.5 the ionic- and electronic conductivities have about the same value ( 1 Ω −1 cm −1 ), d.c. Polarisation measurements yielded the ionic conductivities and the chemical diffusion coefficient, D . The difference between the mobility of the electrons as measured by the Hall effect and from D is explained by means of a model in which only part of the electrons compensating the change in concentration of the mobile ions during the polarisation and relaxation process are free to move.

Electronic and ionic-conduction of solid-solutions ag2-xauxse (0 less-than-or-equal-to x less-than-or-equal-to 0.5)

Die Temperaturen der allotropen Ubergange von Silber-Gold(I)-Chalkogeniden, entsprechend der chemischen Zusammensetzung Ag3AuX2 (X = Se, Te oder S), sind durch differential-thermische Analyse bestimmt worden. Sie finden bei 270°C, 220°C und 320°C bzw. 185°C statt. Bei Abkuhlung sind die Ubergange reversibel und von einer Temperaturhysteresis von 20–30°C begleitet. Nur beim Silbertellurid und beim Silbersulfid mit einem Goldgehalt unterhalb 0,25 molar sind Abweichungen der Ubergangstemperaturen festgestellt worden. Die Ubergangstemperatur dieser Silberchalkogenide erniedrigt sich von 152°C auf 138°C bzw. von 179°C auf 118°C; dies durfte auf eine Versetzung im Kristallgitter der Silberatome durch die Einfuhrung der Goldatome zuruckzufuhren sein, obwohl die Kristallsymmetrie des Silberchalkogenides sich nicht andert.



The temperatures of the allotropic transitions of silver gold(I) chalcogenides corresponding to the chemical composition Ag3AuX2 (X = Se, Te or S) have been determined by means of differential thermal analysis. The transition temperatures are 270, 220 and 320, and 185°C, respectively. All transitions are fully reversible and are accompanied on cooling by a pronounced hysteresis of temperature. Silber telluride as well as silver sulfide containing less than 0.25 moles of gold, on heating present an anomalous thermal behaviour. Although in both compounds the transition temperatures of the non-substituted silver chalcogenides are lowered from 152 to 138°C and from 179 to 118°C, respectively, the formation of a new cristalline phase could not be detected.

Über das thermische Verhalten von Silber‐ und Silber–Gold(I)‐Chalkogeniden

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Über das thermische Verhalten von Silber‐ und Silber–Gold(I)‐Chalkogeniden