Phase Relations in the System Cr-Fe-S

Abstract

The systems Cr-S and Cr-Fe-S are being systematically investigated by silica tube experiments in which vapor is an inherent phase, as well as by collapsible tube experiments under high confining pressures. At 700°C a hexagonal NiAs structure type solid solution, isostructural with high-temperature Fe_1-x S solid solution, exists in the CrS-Cr_0.86S composition range, whereas a rhombohedral structure type solid solution spans the Cr₃S₄-Cr_2.1S₃ composition region. Measurements of the d ₍₁₀₂₎ reflection of Cr_0.93S-Cr_0.86S solid solution members show increasing 2θ values with decreasing chromium content similar to the relations observed in the FeS-Fe_1-x S solid solution series. The crystal structure of Cr₃S₄ is not similar to those of either smythite or greigite (Fe₃S₄); its powder diffraction pattern shows strong structural similarities to that of Cr_2.1S₃, explaining the extensive solid solution between Cr₃S₄ and Cr_2.1S₃. At high temperatures complete solid solution exists between the Fe_1-x S and Cr_1-x S end members. This ternary solid solution manifests itself as a broad field (varying in width from 3.5 to 5.5 wt.% sulfur) spanning the system, and is thus not narrow as reported in the literature. This monosulfide solid solution is intersected at about 700°C by a solvus which results in two coexisting extensive solid solutions. At lower temperatures (650° ± 50 °C) an invariant reaction takes place whereby tie lines between the two monosulfide solid solutions are replaced by tie lines permitting coexistence of daubreelite (Cr₂FeS₄) and metallic αFeCr_ss. Daubreelite occurs in two crystallographic modifications; low-temperature cubic (spinel), and high-temperature hexagonal.