Nickel sulfide is any inorganic compound with the formula NixSy. These compounds range in color from bronze (Ni3S2) to black (NiS2). The nickel sulfide with simplest stoichiometry is NiS, also known as the mineral millerite. From the economic perspective, Ni9S8, the mineral pentlandite, is the chief source of mined nickel. Other minerals include heazlewoodite (Ni3S2), polydymite (Ni3S4), and vaesite (NiS2).[1] Some nickel sulfides are used commercially as catalysts.
Structure
Nickel(II) sulfide
Like many related materials, nickel(II) sulfide, NiS, adopts the nickel arsenide motif. In this structure, nickel is octahedral and the sulfide centers are in trigonal prismatic sites.[2]
| Nickel | Sulfur |
|---|---|
 |
 |
| octahedral | trigonal prismatic |
NiS has two polymorphs. The α-phase has a hexagonal unit cell, while the β-phase has a rhombohedral cell. The α-phase is stable at temperatures above 379 °C (714 °F), and converts into the β-phase at lower temperatures. That phase transition causes an increase in volume by 2–4%.[3][4][5]
Synthesis and reactions
The precipitation of solid black nickel sulfide is a mainstay of traditional qualitative inorganic analysis schemes, which begins with the separation of metals on the basis of the solubility of their sulfides. Such reactions are written:[6]
- Ni2+ + H2S → NiS + 2 H+
Many other more controlled methods have been developed, including solid state metathesis reactions (from NiCl2 and Na2S) and high temperature reactions of the elements.[7]
The most commonly practiced reaction of nickel sulfides involves conversion to nickel oxides. This conversion involves heating the sulfide ores in air:[1]