Allotropes of sulfur

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The two most familiar allotropes of sulfur, rhombic and monoclinic, have the same molecular formula, S8. However, they differ in crystal structure. Using the phase diagram shown in Figure C. you can deduce how to convert either of these allotropes to the other. Notice that rhombic sulfur is the stable allotrope at temperatures below about 95°C. If it is heated to that temperature at... 

At least five high-pressure allotropes of sulfur have been observed by Raman spectroscopy up to about 40 GPa the spectra of which differ significantly from those of a-Sg at high pressures photo-induced amorphous sulfur (a-S) [57, 58, 109, 119, 184-186], photo-induced sulfur (p-S) [57, 58, 109, 119, 184, 186-191], rhombohedral Se [58, 109, 137, 184, 186, 188, 191], high-pressure low-temperature sulfur (hplt-S) [137, 184, 192], and polymeric sulfur (S ) [58, 109, 119, 193]. The Raman spectra of two of these d-lotropes, a-S and S, were discussed in the preceding section. The Raman spectra of p-S and hplt-S have only been reported for samples at high-pressure conditions. The structure of both allotropes are imknown. The Raman spectrum of Se at STP conditions is discussed below. 

Rhombohedral Se was found as a high-pressure allotrope of sulfur above 9-10 GPa by several groups [58, 137, 150, 184, 186, 188, 191]. The pressure dependence of frequencies [137, 150, 184] as well as the kinetics of the transition from p-S to Ss [186] have been investigated systematically by Raman spectroscopy. The pressure dependent frequency shifts of chemically prepared Ss and of high-pressure Ss have been found to be identical [137, 150]. 

II. Well-Established Allotropes of Sulfur A. Molecules with Less Than 6 Atoms... 

A sample of rhombic sulfur, the most stable allotrope of sulfur. 

The most common allotrope of sulfur is orthorhombic a-Ss. At about 95.3°C, ct-Ss transforms to monoclinic such that the packing of Ss molecules is altered and their orientation becomes partly disordered. This leads to a lower density of 1.94 to 2.01 g cm-3, but the dimensions of Ss rings in the two allotropes are very similar. Monoclinic y-Ss also comprises cyclo-Ss molecules, but the packing is more efficient and leads to a higher density of 2.19 g cm-3. It reverts slowly to a-Ss at room temperature, but rapid heating gives a melting point of 106.8°C. 

A new allotrope of sulfur, cyclo-Su, has been isolated as yellow rod-like crystals (mp 113°C) from the reaction of [(tmeda)ZnSe] (tmeda = N,N,N,N/-tetra-methylethylenediamine) with SsCl2. The bond distances vary from 204.7 to 206.1 pm, the bond angles from 104.0° to 109.3°, and the torsion angles in the... 

In the same manner as the existence of other, less familiar oxides of carbon (see 4 above) abound so similarly other oxides of sulfur (see 1 above) are worth noting [20], As well as one of the common allotropes of sulfur, whose form is a puckered eight member ring —... 

Another allotrope of sulfur is formed when the element is melted. This allotrope has no crystalline shape. It looks like a dark brown, thick, melted plastic. 

The following section provides a brief summary of the molecular and crystal structures of the solid allotropes of sulfur mentioned in the Introduction. More specific details about the structures of most of the allotropes can be found in the cited literature. A conclusion concerning the characteristics of the molecular as well as of the crystalline structures of sulfur will be drawn at the end of this section. 

Note that S(rhombic) and P(white) are the most stable allotropes of sulfur and phosphorus, respectively, at 1 atm and 25°C, so their AHJ values are zero. 

Sulfur (S) is second only to carbon in the number of known allotropes formed. The existence of at least twenty-two sulfur allotropes has been demonstrated. The simplest allotrope of sulfur is the violet disulfur molecule, S2, analogous to the dioxygen molecule. Unlike O2, however, S2 does not occur naturally at room temperature and pressure. It is commonly generated in the vapor generated from sulfur at temperatures above 700°C. It has been detected by the Hubble Space Telescope in volcanic eruptions on Jupiter s satellite, lo. 

The same helixes occur, according to Prins, Schenk and Wachters, in the so-called w-sulfur, which is the insoluble residue remaining when chilled viscous sulfur or flowers of sulfur are extracted with carbon disulfide, and which can be produced also in other ways, for instance by hydrolysis of disulfur dichloride. This form is hexagonal, as was discovered in 1938 by Das, who has recently (42) reviewed his X-ray powder studies on allotropes of sulfur. 

The spectra of the unstable solid allotropes of sulfur are not known well. Sg, St, Sg, Si2, and other species (8, 9, 10) form yellow to white solids. Their solution spectra are discussed later. Quenched liquid sulfur or vapor produces yellow polymeric sulfur. Its spectrum shows a shoulder at 360 nm, as shown later. If liquid sulfur or sulfur vapor is quenched rapidly to 77 °K or below, deep colored films are formed. The spectrum of red sulfur produced by quenching a boiling liquid film is shown in Figure 2. The color is a result of three spectral features the absorption edge of polymeric sulfur, an absorption peak at 400 nm which is caused by the lowest allowed electronic transition of Sa, and an absorption peak at 550 nm which is a result of S4. The spectrum of trapped vapor consists of several broad peaks it changes with the composition of the vapor source and depends on speed of deposition and many other factors. The deep color of all trapped metastable species fades at — 90°C to deep yellow. Above this temperature, the spectrum shows only polymeric sulfur and Sg. 

Sulfur exists in a number of allotropic forms that differ primarily in their crystal structures. The molecular form of sulfur consists of eight sulfur atoms in a ring—denoted S. Melting sulfur and then cooling it rapidly creates a plastic form of sulfur. The powdered sulfur often found in school chemistry laboratories is obtained by sublimation and is known as flowers of sulfur. Sulfur is a nonconductor of electricity and is insoluble in water. The allotropes of sulfur can take on a wide variety of forms, as shown in the table on page 113. 

Allotropes of an element are dilferent structural modifications of that element. Allotropes of sulfur include cyclic... 

The phase diagram of sulfur is shown here, (a) How many triple points are there (b) Monoclinic and rhombic are two allotropes of sulfur. Which is more stable under atmospheric conditions (c) Describe what happens when sulfur at 1 atm is heated from 80°C to 200°C. 

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