Selenium monoclinic

Figure 20. Se -edge XANES spectra of (A) Se-containing model compounds (a) Se042 (aq), (b) SeC>32 (aq), (c) selenomethionine (aq), (d) elemental (red) selenium (monoclinic) (B) (a) Se in Kesterson Reservoir soil at depth of 0 to 0.05 m (340 ppm Se), (b) Se in Kesterson Reservoir soil at depth of 0.05 to 0.15 m (40 ppm Se), (c) Se in mushroom (Agaricus bernardii) collected adjacent to the reservoir (500 ppm Se). Linear combination fits of the Se K-XANES of the three Kesterson samples resulted in the following components (a) 97% elemental Se + 3% aqueous selenite (b) 86% elemental selenium + 14% aqueous selenite (c) 71% selenomethionine + 11% aqueous selenite + 18% selenocystine. Data were taken on SSRL beam line 4-3. (after Pickering et al. 1995)...

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

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. 

Suzuki, T. M., Tanco, M. L., Tanaka, D. A. P., Yokoyama, T., Matsunaga, H., Yokoyama, T., Adsorption characteristic and removal of oxo-anions of arsenic and selenium on the porous polymers loaded with monoclinic hydrous zirconium oxide, Sep. Sci. Technol., 36, 2001,103-111. 

Preliminary X-ray investigations have been followed by several detailed crystal structure determinations of various sulfur-selenium phases with ring sizes of eight and twelve. The unit cell parameters of the eight-membered species are given in Table 1. The unit cell parameters of monoclinic y-sulfur and of monoclinic a-selenium are included for comparison. 

It can clearly be seen from the crystal data of Table 1 that the phases form two isomorphic series with a break at about 50 mol-% of selenium. The sulfur-rich species crystallize in the monoclinic y-sulfur lattice and the selenium-rich species in the monoclinic a-selenium lattice (see Fig. 1). 

Fig. 1 a and b. The unit cell contents of (a) monoclinic y-sulfur (a horizontal, c vertical) and (b) monoclinic a-selenium (c horizontal, b vertical) as completed to show full molecules. Atomic coordinates of monoclinic y-sulfur are according to Wata-nabe and those of monoclinic a-selenium according to Cherin and Unger... 

Intermolecular distances in these sulfur-selenium phases are comparable to those observed for monoclinic y-sulfur and monoclinic ot-selenium The shortest distances in both isomorphic series are much shorter than the sum of the van der Waals radii of the elements. In the isomorphic series of the sulfur-rich species the shortest distanee between the molecules gets shorter with increasing selenium content of the phase but remains effectively constant in the selenium-rich series. 

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

Solid sulfur trioxide exists in two well-defined modifications. Orthorhombic y-SOs consists of cyclic trimers (Figure 12.38a)/ which are converted by traces of water to the one-dimensional polymeric structure of monoclinic jl-SOi, (Figure 12.38b). In contrast, selenium trioxide forms a cyclic tetramer (Figure 12.38c). ... 

Solid Allotropes.—Both amorphous and crystalline varieties of selenium occur. Amorphous selenium is best known as the vitreous and the finely divided brick-red forms, which are frequently described as two distinct allotropes they are, however, identical. The crystalline allotropes include several monoclinic varieties, red to brown in colour, as well as the so-called metallic selenium. 

Both of the foregoing varieties of amorphous selenium are somewhat soluble in carbon disulphide, but selenium chloride, carbon diselenide and methylene iodide are better solvents.4 On account of its finer state of division the red form appears more soluble. Discordant results are easily obtained with such solutions because of the tendency to change, especially on warming, into the less soluble crystalline variety —generally the monoclinic form. 

Crystalline Varieties.—(1) Monoclinic Selenium is obtained when a carbon disulphide solution of amorphous selenium is allowed to crystallise, for example by evaporation 8 or by keeping red amorphous or vitreous selenium in contact with carbon disulphide. In the latter case the... 

Several varieties of monoclinic selenium are obtainable in this way reddish-brown leaflets, more deeply coloured granules, or stout prisms of still deeper colouring.2... 

Monoclinic selenium has a density of 4-44 3 at 0° C. It is only sparingly soluble in carbon disulphide, giving a red solution. When heated rapidly it melts at 170° to 180° C., but transformation into metallic selenium commences to occur slowly near 120° C.4 The liquid obtained by rapid heating passes into the vitreous condition when cooled. 

The liquefied substance solidifies at —64° C. and boils at —42° C. under 760 mm. pressure, the critical temperature being +138° C.2 One litre of the gas at N.T.P. weighs 3-6715 grams. The solubility in water at 4° C. is 3-77 volumes per unit volume of solvent, whilst at 22-5° C. water dissolves 2-70 times its own volume of the gas.3 The gas is also soluble in molten selenium, being freed on solidification.4 The combination of hydrogen gas and amorphous selenium is accompanied by an absorption of 16-0 Cals, per gram-molecule of hydrogen selenide produced with the use of the more stable monoclinic and metallic forms, the values are 17-0 and 17-4 Cals, respectively.5... 

Selenium dioxide is an exothermic compound, the heat of formation being 57-2 Calories per gram-molecule, referred to vitreous selenium, whilst the value is somewhat less for the monoclinic element and less still for metallic selenium.1... 

The production of the hydrogen sulphide serves to prevent the selenate being oxidised to the ferric state. There may also be a slight secondary reaction resulting in the precipitation of both sulphur and selenium according to the equation on p. 335. From the solution unstable monoclinic crystals of hydrated ferrous selenate, FeSe04.7H20, may be deposited. 

We have discussed structures of O2, S6, and Ss- Now we consider Se, Te, and Po. Six crystalline forms of selenium have been reported ot-Se (stable under normal conditions), a-monoclinic, and (3-monoclinic Se, and three cubic forms deposited as thin films by vacuum evaporation. Metallic a-Se is trigonal, but also described as a distorted simple cubic structure, 3PO, similar to the structure of Te with more distortion for Se. There are infinitive helices parallel to the c axis. The space groups of a-Se are Dg, P3i21 and Dg, P3221 for the other enantiomorph (see Section 10.1.3). 

Red monoclinic selenium exists in three forms, each containing Ses rings with the crown conformation of Sg (Fig. 16.4.1). Vitreous black selenium, the ordinary commercial form of the element, comprises an extremely complex and irregular structure of large polymeric rings. 

The crown-shaped molecule Se8 has long been known to crystallize in two monoclinic lattices termed a- and jS-Se8, respectively (see Table II). Crystals of various sizes have been obtained from CS2 solutions prepared by dissolution of either red amorphous Se (22) or of vitreous selenium in CS2 and subsequent cooling or evaporation of the solvent (3a,b, 4a, 5). Red amorphous Se is readily soluble in CS2, but since both vitreous and red amorphous Se do not contain more than small amounts of Se8, the dissolution must be accompanied by a chemical interconversion. It has, however, been reported (26) that the dissolution of red amorphous Se in CS2 requires illumination with photons of energies in excess of 2.3 eV, ambient room light levels being sufficient. 

The spontaneous but slow crystallization of monoclonic selenium (Se8) from red amorphous Se, prepared by quenching of selenium vapor (1000°C) in liquid nitrogen, depends on the storage temperature of the samples. Below 303 K, red amorphous Se(vap) is completely transformed into the monoclinic phase, whereas above 303 K transformation into the metallic phase takes place. (The relative Se8 content of the material can be determined by DSC.) In contrast, chemically prepared red amorphous Se(redn) is transformed only into the metallic phase, even below 303 K (21). 

Raman and infrared spectra of a-monoclinic selenium have been reported (3c,d, 4b, 14, 37) the low-temperature Raman spectrum is shown in Fig. 5. It differs sufficiently from that of Se6 for both compounds to be detected in this way in mixtures with each other (15). The observed wave numbers have been assigned to the 11 fundamental modes of the Seg molecule, assuming Z)4d symmetry (see Table V). The ft2 and e modes are infrared active, and the ai, e2, and e3 vibrations are Raman active. The wave number of the inactive b fundamental has been estimated from the second-order Raman spectrum. 

Fig. 5. Low-temperature Raman spectra (-105 5°C) of monoclinic a-Se8 (a) and of red amorphous selenium prepared by reduction of Se02 (b) (14).

Several forms of selenium are known also. In addition to two red (monoclinic) and a grey (hexagonal) form (with melting point 217° C), there have been reported several forms of amorphous selenium (both red and black). The grey form (and presumably also the amorphous forms) of selenium consist of zig-zag chains of bound selenium atoms, but one of the red forms has been shown to contain Se8 rings. The grey form of selenium has become especially important in recent years the sharp increase in its electrical conductivity upon exposure to light makes it a valuable material for use in photoconductive cells. 

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