Selenium Hexagonal

INTRODUCTION This data sheet summarizes properties of single crystal hexagonal selenium. 

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ISe h - Selenium hexagonal system, the structure represents the infinite layers where each atom of selenium has two neighbors. 

Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red crystalline hexagonal selenium, the most stable variety, is a metallic gray. 

Figure 16.1 Structures of various allotropes of selenium and the structure of crystalline tellurium (a) the Seg unit in a- fi- and y-red selenium (b) the helical Se chain along the c-axis in hexagonal grey selenium (c) the similar helical chain in crystalline tellurium shown in perspective and (d) projection of the tellurium structure on a plane perpendicular to the c-axis.

Trigonal selenium is variously called metallic gray or black selenium and occurs in lustrous hexagonal crystals, which melt at 220.5 °C. Its structure, which has no sulfur analogue, consists of infinite, unbranched helical chains. Its density, 4.82 g cm , is the highest of any form of the element. Trigonal selenium is a semiconductor (intrinsic p-type with a rather indirect transition at about 1.85 eV [5]), and its electronic and photoelectric properties are the basis for many industrial uses of this element. 

Fig. 3.8 XRD patterns (CuKc( source) showing a rich in selenium (x > 0.6) CdSej Tei-jt electrodeposited film, which adopts the hexagonal CdSe wurtzite structure by annealing at 520 °C (b). The as-deposited (from a typical acidic solution) film is rather amorphous (a). Segregation of a Te phase is observed in the solid. (Reprinted from Bouroushian et al. [137], Copyright 2009, with permission from Elsevier)...

SeS3 (m.p. 111 °C) is said to be formed in the reaction of various N-benzylthiamides (R—CS—NH—CHj -CgHj) and selenium dioxide in ethanol. After repeated recrystallization from carbon disulfide long red hexagonal prisms of SeSj are obtained from the crude product... 

Amorphous forms exhibit two colors, occurring as a red powder of density 4.26g/cm3 that has a hexagonal crystal structure and a black vitreous solid of density 4.28g/cm3. The red amorphous selenium converts to the black form on standing. Amorphous selenium melts at 60 to 80°C insoluble in water reacts with water at 50°C when freshly precipitated soluble in sulfuric acid, benzene and carbon disulfide. 

Crystalline selenium exhibits two monochnic forms an alpha form constituting dark red transparent crystals, density 4.50 g/cm. The alpha form converts to a metastable beta form of hexagonal crystal structure when heated to about 170°C. Both the crystalline forms are insoluble in water soluble in sulfuric and nitric acids very slightly soluble in carbon disulfide. Also, both the crystalline forms convert into gray metallic modification on heating. 

The gray metallic form of selenium is its most stable modification. It constitutes lustrous gray to black hexagonal crystals density 4.18 g/cm at 20° melts at 217°C soluble in sulfuric acid and chloroform very slightly soluble in carbon disulfide insoluble in alcohol. 

The selenium filtered from the reaction mixture is refluxed with 31. of 95% ethanol for 1 hour, which converts the red amorphous form to the gray hexagonal form and frees it of organic matter. This metallic selenium is removed by filtration, washed with water, and converted to the dioxide by the method of Baker and Maxson.2... 

In the native condition the element is occasionally found as hexagonal prismatic crystals, which are nearly pure. The foreign metals present, usually only in small quantity, may be gold, silver, copper and iron. Free tellurium is also present to the extent of approximately 0-17 per cent, in the red sulphur of Japan, which also contains about 6 per cent, of selenium and 1 per cent, of arsenic.8 Traces of tellurium may be found in Spanish pyrites.9... 

The luminescent properties can be influenced by the nature of the activators and coactivators, their concentrations, the composition of the flux, and the firing conditions. In addition, specific substitution of zinc or sulfur in the host lattice by cadmium or selenium is possible, which also influences the luminescent properties. Zinc sulfide is dimorphic and crystallizes below 1020 °C in the cubic zinc-blende structure and above that temperature in the hexagonal wurtzite lattice. When the zinc is replaced by cadmium, the transition temperature is lowered so that the hexagonal modification predominates. Substitution of sulfur by selenium, on the other hand, stabilizes the zinc-blende lattice. 

Structure of a-selenium (or tellurium) (a) side view of Se (or Te ) helical chain (b) viewed along die helices die hexagonal unit cell and the coordination environment about one atom is indicated. 

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