The structure of high-temperature heazlewoodite Ni3±xS2 is not totally clear in literature. In addition, there is a dispute about whether a division of the Ni3±xS2 phase field into two solid solutions, ß1 (around Ni3S2) and ß2 (around Ni4S3), occurs or whether a wide monophase field of Ni3±xS2 exists. In the framework of a comprehensive assessment of the nickel-sulfur and iron-nickel-sulfur systems [1] the data have recently been reviewed and, in view of the contradictory situation in the literature, a single-phase model was applied successfully over the entire homogeneity range of high-temperature heazlewoodite. Furthermore, in this paper a two‑sublattice approach using the Compound Energy Formalism is proposed for a ‘second’ high-temperature heazlewoodite phase around the composition Ni4S3. The question of a possible division of the Ni3±xS2 phase field is discussed on the basis of a consistent thermodynamic description of the complete system nickel-sulfur. High-temperature heazlewoodite dissolves substantial amounts of iron and forms an extended solid solution in the ternary system iron-nickel-sulfur. The extension of the proposed model to a ternary solution phase offers new possibilities to interpret complex experimental phase equilibria data in the ternary system iron-nickel-sulfur.
[1] WALDNER P. & PELTON A.D. "Thermodynamic Modeling of the Fe-Ni-S System", CALPHAD XXII Conference, La Malbaie, QC, Canada, May 25-30, 2003 (Book of Abstr., p.47).