Fahlore thermochemistry: Gaps inside the (Cu,Ag)10(Fe,Zn)2(Sb,As)4S13 cube

Abstract

Possible topologies of miscibility gaps in arsenian (Cu,Ag)10(Fe,Zn)2(Sb,As)4S13 fahlores are examined. These topologies are based on a thermodynamic model for fahlores whose calibration has been verified for (Cu,Ag)10(Fe,Zn)2Sb4S13 fahlores, and conform with experimental constraints on the incompatibility between As and Ag in (Cu,Ag)10(Fe,Zn)2(Sb,As)4S13 fahlores, and with experimental and natural constraints on the incompatibility between As and Zn and the nonideality of the As for Sb substitution in Cu10(Fe,Zn)2(Sb,As)4S13 fahlores. It is inferred that miscibility gaps in (Cu,Ag)10(Fe,Zn)2As4S13 fahlores have critical temperatures several °C below those established for their Sb counterparts (170 to 185°C). Depending on the structural role of Ag in arsenian fahlores, critical temperatures for (Cu,Ag)10(Fe,Zn)2(Sb,As)4S13 fahlores may vary from comparable to those inferred for (Cu,Ag)10(Fe,Zn)2As4S13 fahlores, if the As for Sb substitution stabilizes Ag in tetrahedral metal sites, to temperatures approaching 370°C, if the As for Sb substitution results in an increase in the site preference of Ag for trigonal-planar metal sites. The latter topology is more likely based on comparison of calculated miscibility gaps with compositions of fahlores from nature exhibiting the greatest departure from the Cu10(Fe,Zn)2(Sb,As)4S13 and (Cu,Ag)10(Fe,Zn)2Sb4S13 planes of the (Cu,Ag)10(Fe,Zn)2(Sb,As)4S13 fahlore cube.