Metastable Nickel Sulfides with Composition Close to Ni7S6—Stability and Structural Properties

Abstract

Four well-defined metastable phases with composition close to Ni7S6are described. These phases are structurally closely related to the high-temperature phase Ni7±xS6, which is formed eutectoidally from Ni9S8and Ni3S2at ∼675 K. The relative stability and a preliminary structural description of the metastable phases have been obtained from X-ray diffraction, transmission electron microscopy, calorimetry, and thermal analysis. Conditions for preparation of the different metastable phases are provided. On rapid quenching of Ni7±xS6in ice + water + NaCl, an as-quenched state is obtained. This state is rather unstable and transforms on storing at room temperature to the metastable phase denoted αivand subsequently to αiii. The actual phase sequence observed during heating of these modifications depends strongly on the thermal history of the sample and especially on the heating rate used in the experiments. Either of the two quasi-reversible phase sequences αiv⇔ αii⇔ αi⇔ Ni7±xS6or αiii⇔ αi⇔ Ni7±xS6is observed, where αi, αii, αiii, and αivdenote the four well-defined metastable phases. The as-quenched samples appear rather similar to αi. Furthermore, an exothermal phase transformation is observed for αiii. It has not, however, been possible to relate this transformation to structural changes.