Gibbs Energy Modeling of Digenite and Adjacent Solid-State Phases
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INTRODUCTION
DIGENITE is the dominating intermediate solid solution of the Cu-S system with a broad phase field covering a compositional range approximately from the Cu2S stoichiometry up to 36.6 at. pct S. According to the review of Chakrabarti and Laughlin,[1] digenite melts congruently at 1403 K ± 2 K (1130 C ± 2 C) and is stable over a wide temperature range down to 345 K (72 C ± 3 C). At 1 bar total pressure, the Cu-S system contains two stoichiometric compounds, anilite Cu1.75S and covelitte CuS. Digenite as one of six condensed solution phases shows solid-state equilibria with the Cu alloy phase, high- and low-temperature chalcocites with a Cu-rich limit of Cu2S stoichiometry as well as with djurleite around the composition Cu1.95S. On both sides of digenite, the liquid phase separates into two miscibility gaps above 1378 K (1105 C) at copper-rich compositions and above 1086 K (813 C) at higher sulfur compositions. For the digenite solid solution, a C1-type crystal structure (Pearson symbol cF12, space group Fm3m, CaF2 prototype) is reported. While the close-packed structure of sulfur is well defined in the literature, the crystallographic characterization of the positions of Cu species within the close-packed sublattice of S has been a long-standing and challenging
PETER WALDNER is with the Department of General, Analytical and Physical Chemistry, University of Leoben, Leoben 8700, Austria. Contact e-mail: [email protected]. Manuscript submitted 12 January 2017. Article published online May 25, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
problem (Rahlfs,[2] Morimoto and Kullerud,[3] Will et al.,[4] Lukashev et al.[5]). Modeling studies on digenite were subject of several publications in the literature as those of Rau,[6–8] first assuming imperfections in the lattice, such as neutral and negatively charged copper vacancies combined with their associations, and then interpreting digenite as a solution of Cu2S and CuS components with copper interstitials. Nagamori[9] described digenite as an ionic crystal consisting of Cu+, Cu2+, Cu0, and neutral vacancies on the tetrahedral sites within the face-centered cubic lattice with sulfur species as S2- and a constant and low amount of neutral vacancies. A thermodynamic equation of state describing the structure of digenite to be of cubic C1-type was given by Sharma and Chang.[10,11] Within an assessment of a partial phase diagram of the copper–sulfur system, digenite was treated as a stoichiometric phase by Dinsdale et al.[12] Proposing a defect model for digenite on the basis of neutral and charged copper vacancies, conductivity data determined by Pareek et al.[13] were interpreted. Thermodynamic calculations on the stability of Cu2S in low carbon steel were performed by Lee et al.[14] using the compound energy formalism with a three-sublattice approach for digenite. On the basis of the antifluorite structure, a similar model limited to two sublattices was derived by Waldner and Sitte.[15] The digenite solid solution was described by Shishin an
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