Chalcogenides —

If you move left one column in the periodic table from the halides, the chalcogenides need two electrons to complete their valence shell, and thus can bond to the surface and each other simultaneously. This appears to account for much of the interesting surface chemistry of chalcogenide atomic layers. Chalcogenides, including oxides (corrosion), are some of the most studied systems in surface chemistry. The oxides are clearly the most important, but significant amounts of work have been done with sulfur, selenium and tellurium. 

There are great similarities between the atomic layer structures formed by Te, Se and S on the low index planes of Au [238]. Table 2 shows a listing of structures 

Au(lll) Coverage Sulfur Comments Refs. Selenium Comments Refs. Tellurium Comments Refs. 

Manassen, and R. Tenne,, 4m. Chem. Soc.. Symp. Ser., 1981, 146 (Photoeffects at Semiconductor Electrolyte Interfaces), 369. 

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Svane A, Temmerman W and Szotek Z 1999 Theory of pressure-induced phase transitions in cerium chalcogenides Phys. Rev. B 59 7888... 

Lover T efa/1997 Electrospray mass spectrometry of thiophenolate-capped clusters of CdS, CdSe and ZnS and cadmium and zinc thiophenolate complexes observation of fragmentation and metal, chalcogenide and ligand exchange processes Inorg. Chem. 36 3711... 

Selenium chalcogenide glasses exhibit good infrared transmission properties. These are used as lenses (ZnSe, CdSe) in laser apphcations and have potential applications in fiber optics (qv) and in data storage and retrieval. 

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. 

Aqueous solutions have low conductivities resulting from extensive complex ion formation. The haUdes, along with the chalcogenides, are sometimes used in pyrotechnics to give blue flames and as catalysts for a number of organic reactions. 

NEW COMPLEX CHALCOGENIDES AS SENSITIVE MATERIALS FOR ION SELECTIVE ELECTRODES... 

Betyllium, because of its small size, almost invariably has a coordination number of 4. This is important in analytical chemistry since it ensures that edta, which coordinates strongly to Mg, Ca (and Al), does not chelate Be appreciably. BeO has the wurtzite (ZnS, p. 1209) structure whilst the other Be chalcogenides adopt the zinc blende modification. BeF2 has the cristobalite (SiOi, p. 342) structure and has only a vety low electrical conductivity when fused. Be2C and Be2B have extended lattices of the antifluorite type with 4-coordinate Be and 8-coordinate C or B. Be2Si04 has the phenacite structure (p. 347) in which both Be and Si... 

The oxides MO are best obtained by calcining the carbonates (pp. 114 and 122) dehydration of the hydroxides at red heat offers an alternative route. BeO (like the other Be chalcogenides)... 

The chalcogenides of Ga, In and T1 are much more numerous and at least a dozen different structure types have been established by X-ray... 

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