Molybdenum disulfide

Molybdenum disulfide [molybdenum(IV) sulfide, MoS₂] is an inorganic compound that exists in nature in the mineral molybdenite. Its crystals have a hexagonal layered structure (shown) that is similar to graphite.

In 1957, Ronald E. Bell and Robert E. Herfert at the now-defunct Climax Molybdenum Company of Michigan (Ann Arbor) prepared what was then a new rhombohedral crystalline form of MoS₂. Rhombohedral crystals were subsequently discovered in nature.

Like most mineral salts, MoS₂ has a high melting point, but it begins to sublime at a relatively low 450 ºC. This property is useful for purifying the compound.

Because of its layered structure, hexagonal MoS₂, like graphite, is an excellent “dry” lubricant. It and its cousin tungsten disulfide can be used as surface coatings on machine parts (e.g., in the aerospace industry), in two-stroke engines (the type used for motorcycles), and in gun barrels (to reduce friction between the bullet and the barrel).

Unlike graphite, MoS₂ does not depend on adsorbed water or other vapors for its lubricant properties. It can be used at temperatures as high as 350 ºC in oxidizing environments and up to 1100 ºC in nonoxidizing environments. Its stability makes it useful in high-temperature applications in which oils and greases are not practical.

In addition to its lubricating properties, MoS₂ is a semiconductor. It is also known that it and other semiconducting transition-metal chalcogenides become superconductors at their surfaces when doped with an electrostatic field.

The mechanism of superconductivity was uncertain until 2018, when Andrea C. Ferrari at the University of Cambridge (UK) and colleagues there and at the Polytechnic Institute of Turin (Italy) reported that a multivalley Fermi surface is associated with the superconductivity state in MoS₂. The authors believe that “this [Fermi surface] topology will serve as a guideline in the quest for new superconductors.”