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, tellurides, and polonides, rather than oxides. Many metal ores exist as chalcogenides. Photoconductive 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 chalcogenides 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 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−, Te2−, and Po2−. In fact, transition metal chalcogenides are highly covalent, not ionic, as indicated by their semiconducting properties. Metal-rich chalcogenides In most of their chalcogenides, transition metals adopt oxidation states of II or greater. Nonetheless, several examples exist where the metallic atoms far outnumber the chalcogens. Such compounds typically have extensive metal-metal bonding. Monochalcogenides Metal monochalcogenides have the formula ME, where M = a transition metal and E = S, Se, Te. They typically crystallize in one of two motifs, named after the corresponding forms of zinc sulfide. In the zinc blende structure, the sulfide atoms pack in a cubic symmetry and the Zn2+ ions occupy half of the tetrahedral holes. The result is a diamondoid framework. The main alternative structure for the monochalcogenides is the wurtzite structure wherein the atom connectivities are similar (tetrahedral), but the crystal symmetry is hexagonal. A third motif for metal monochalcogenide is the nickel arsenide lattice, where the metal and chalcogenide each have octahedral and trigonal prismatic coordination, respectively. This motif is commonly subject to nonstoichiometry. Important monochalcogenides include some pigments, notably cadmium sulfide. Many minerals and ores are monosulfides. Dichalcogenides Metal dichalcogenides have the formula ME2, where M