COMPOUNDS CHALCOGENIDES

The chalcogenides are binary compounds of a chalcogen (i.e., the elements of Group Ilb zinc, cadmium, mercury) with a less electropositive element, such as those of Group VIb (oxygen, sulfur, selenium, and tellurium). This section covers the sulfides, selenides, andtellurides. Oxides are reviewed above in Ch. 11. Most of the chalcogenides have useful optical characteristics and their applications are usually found in optics. 

The II-VI compounds now being produced by CVD include ZnSe, ZnS, ZnTe, CdS, HgTe, CdMnTe, and HgCdTe. CdTe, ZnS, and ZnSe are the most important from an industrial standpoint. Selected properties of these three chalcogenides are summarized in Table 12.4. 

Note Zinc sulfide (ZnS) has higher mechanical properties and better erosion resistance than zinc selenide but its optical transmission is not as good.  

The metals of group Ilb (Zn, Cd, Hg) have low vaporization temperatures (Zn 907°C, Cd 756°C, Te 990°C). This favors the reaction of the vaporized metal with an alkyl of the group VI element at a temperature range of325-350°C, as shown in the following examples  

Other II-VI compounds are deposited by the reaction of the Group VIb hydride with the Group lib vaporized metal 

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The CVD of Ceramic Materials Borides, SUicides, III—V Compounds and II—VI Compounds (Chalcogenides)... 

Data for some II-VI compounds (chalcogenides) are shown in Figure 5.5. 

Keywords Resistive switching - Mott Insulator - Lacunar spinel - Cluster compound -Chalcogenide. 

The cadmium chalcogenide semiconductors (qv) have found numerous appHcations ranging from rectifiers to photoconductive detectors in smoke alarms. Many Cd compounds, eg, sulfide, tungstate, selenide, teUuride, and oxide, are used as phosphors in luminescent screens and scintiUation counters. Glass colored with cadmium sulfoselenides is used as a color filter in spectroscopy and has recently attracted attention as a third-order, nonlinear optical switching material (see Nonlinear optical materials). DiaLkylcadmium compounds are polymerization catalysts for production of poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVA), and poly(methyl methacrylate) (PMMA). Mixed with TiCl, they catalyze the polymerization of ethylene and propylene. 

The binary compounds of the Group 13 metals with the elements of Group 15 (N, P, As, Sb, Bi) are stmcturally less diverse than the chalcogenides just considered but they have achieved considerable technological application as III-V semiconductors isoelectronic with Si and Ge (cf. BN isoelectronic with C, p. 207). Their stmctures are summarized in Table 7.10 all adopt the cubic ZnS stmcture except the nitrides of Al, Ga and In which are probably more ionic (less covalent or metallic) than the others. Thallium does not form simple compounds... 

All three elements combine readily with most metals and many non-metals to form binary chalcogenides. Indeed, selenides and tellurides are the most common mineral forms of these elements (p. 748). Nonstoichiometry abounds, particularly for compounds with the transition elements (where electronegativity differences are minimal and variable valency is favoured), and many of the chalcogenides can be considered... 

Chalcogenides of Cd are similar to those of Zn and display the same duality in their structures. The sulfide and selenide are more stable in the hexagonal form whereas the telluride is more stable in the cubic form. CdS is the most important compound of cadmium and, by addition of CdSe, ZnS, HgS, etc., it yields thermally stable pigments of brilliant colours from pale yellow to deep red, while colloidal dispersions are used to colour transparent glasses. 

The oxide (p. 1209), chalcogenides (p. 1210) and halides (p. 1211) have already been described. Of them, the only ionic compound is HgF2 but other compounds in which there is appreciable charge separation are the hydrated salts of strong oxoacids, e.g. the nitrate, perchlorate, and sulfate. In aqueous solution such salts are extensively hydrolysed (HgO is only very weakly basic) and they require acidification to prevent the formation of polynuclear hydroxo-bridged species or the precipitation of basic salts such as Hg(OH)(N03) which contains infinite zigzag chains ... 

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