Metal dichalcogenide
A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term chalcogenide is more commonly reserved for sulfides, selenides, and tellurides, rather than oxides. Many metal ores exist as chalcogenides. chalcogenide glasses are used in xerography. Some pigments and catalysts are also based on chalcogenides. The metal dichalcogenide MoS2 is a common solid lubricant.
Alkali metal and alkaline earth monochalcogenides are salt-like, being colourless and often water-soluble. The sulfides tend to undergo hydrolysis to form derivatives containing bisulfide (SH−) anions. The alkali metal chalcogenides often crystallize with the antifluorite structure and the alkaline earth salts in the sodium chloride motif.
Transition metal chalcogenides occur with many stoichiometries and many structures. Most common and most important technologically, however, are the chalcogenides of simple stoichiometries, such as 1:1 and 1:2. Extreme cases include metal-rich phases (e.g. Ta2S), which exhibit extensive metal-metal bonding, and chalcogenide-rich materials such as Re2S7, which features extensive chalcogen-chalcogen bonding.
For the purpose of classifying these materials, the chalcogenide is often viewed as a dianion, i.e., S2−, Se2−, and Te2−. In fact, transition metal chalcogenides are highly covalent, not ionic, as indicated by their semiconducting properties.
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