Sulfur-selenium isomorphous substitution and morphotropic transition in the Ag_3Au(Se,S)_2 series

摘要

Gold-silver sulfoselenides of the series Ag3AuSexS2-x (x = 0.25; 0.5; 0.75; 1; 1.5) were synthesized from melts on heating stoichiometric mixtures of elementary substances in evacuated quartz ampoules. According to X-ray single-crystal analysis, compound Ag_3Au_1Se_(0.5)S_(1.5) has the structure of gold-silver sulfide Ag3AuS2 (uytenbogaardtite) with space group R3c. The volume of this compound is 1.5% larger than that of the sulfide analog. According to powder X-ray diffraction, compounds Ag_3AuSe_(0.25)S_(1.75) and Ag_3AuSe_(0.75)S_(1.25) also show trigonal symmetry. Compounds Ag_3AuSeS and Ag_3AuSe_(1.5)S_(0.5) are structurally similar to the low-temperature modification of gold-silver selenide Ag_3AuSe_2 (fischesserite) with space group I4132. These data suggest the existence of two solid solutions: petzite-type cubic Ag_3AuSe_2-Ag_3AuSeS (space group I4132) and trigonal Ag_3AuSe_(0.75)S_(1.25)-Ag_3AuS_2 (space group R3c).It was found that fischesserite from the Rodnikovoe deposit (southern Kamchatka) contains 3.5-4 wt.% S. At the Kupol deposit (Chukchi Peninsula), fischesserite contains up to 2.5 wt.% S and uytenbogaardtite contains up to 5.3 wt.% Se. At the Ol'cha and Svetloe (Okhotskoe) deposits (Magadan Region), uytenbogaardtite contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data on the compositions of silver-gold selenides and sulfides from different deposits were summarized and analyzed. Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS.