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Chalcogenide glass
Chalcogenide glass (hard "ch" as in "chemistry") is a glass containing one or more chalcogenide elements (not counting oxygen). The name chalcogenide originates from the Greek word "chalcos" meaning ore and "gen" meaning formation, thus the term chalcogenide is generally considered to mean ore former.〔William B. Jensen, "A note on the term chalcogen", Journal of chemical education, 74:1063, 1997〕 These are three elements in Group 16 in the periodic table: sulfur, selenium and tellurium. Such glasses are covalently bonded materials and may be classified as network solids; in effect, the entire glass matrix acts as an infinitely bonded molecule. Although polonium is chemically a chalcogenide as well, it is not used in chalcogenide glasses because of its strong radioactivity and high price. Oxygen is also a group 16 element, but it is not considered a chalcogenide. Though oxide materials are the oldest known glass forming systems it has become more traditional to treat them separately from more recently discovered chalcogenide compounds. Scientifically oxide materials behave rather differently from other chalcogenides, in particular their widely different band gaps contribute to very dissimilar optical and electrical properties. Chalcogenides can exist naturally as minerals; two of the most well-known being FeS2 (pyrite) and AuTe2 (calaverite).
The classical chalcogenide glasses (mainly sulfur-based ones such as As-S or Ge-S) are strong glass-formers and possess glasses within large concentration regions. Glass-forming abilities decrease with increasing molar weight of constituent elements; i.e., S > Se > Te. The semiconducting properties of chalcogenide glasses were revealed in 1955 by B.T. Kolomiets and N.A. Gorunova from Ioffe Institute, USSR. This discovery initiated numerous researches and applications of this new class of semiconducting materials.
Modern chalcogenide compounds like AgInSbTe and GeSbTe, widely used in rewritable optical disks and phase-change memory devices, are fragile glass-formers; by applying heat, they can be switched between an amorphous (glassy) and a crystalline state, thereby changing their optical and electrical properties and allowing the storage of information.
==Chemistry==
The model of a binary glass-forming chalcogenide is considered to be analogous to silica; there are two group 16 chalcogen elements bonded to a single group 14 element. Another common class of chalcogenides have glass-forming regions where three chalcogens are bonded to two group 15 elements. Most stable binary chalcogenide glasses are compounds of a chalcogen and a group 14 or 15 element. This allows a wide range of atomic ratios. Ternary glasses allow a larger variety of atoms to be incorporated into the glass structure, thus giving greater engineering capacity.〔M.C. Flemings, B. Ilschner, E.J. Kramer, S. Mahajan, K.H. Jurgen Buschow and R.W. Cahn, Encyclopedia of Materials: Science and Technology, Elsevier Science Ltd, 2001.〕 Although chalcogenides can exist over a wide range of compositions, not all of which exist in glassy form, it is often possible to find materials with which these non-glass-forming compositions can be alloyed in order to form a glass. An example of this is gallium sulphide-based glasses. Gallium sulphide on its own is not a known glass former; however, it readily bonds with sodium or lanthanum sulphides to form a glass, gallium lanthanum sulphide (GLS). Amorphous chalcogenide materials can be broadly classed by the type of atoms to which they bond to form amorphous systems. One of the more well-known chalcogenide glasses is based on arsenic trisulphide, an example of a stable binary glass which preferentially exists in a glassy phase. In contrast, compounds based on heavier chalcogenides—for example, tellurium-based materials—are more likely to exist as a crystal.
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