Polymorphism of sulfur

We now consider other homocyclic polymorphs of sulfur containing 6 - 20 S atoms per ring. A rhombohedral form, t-sulfur, was first prepared by M. R. Engel in 1891 by the reaction of concentrated HCl on a saturated solution of thiosulfate HS2O3 at 0°. It was shown to be... 

The ring adopts the chair Ibrm and its dimensions are compared with those of other polymorphs in Table 15.5. Note that cyc/o-S has the smallest bond angle and dihedial angle of all poly-sulfur species for which data are available and this, Uigether with the small hole at the centre of the molecule and the efficient packing within the crystal, lead to the highest density of any known polymorph of sulfur (Table 15.6). 

VA Fig. 16.17 The structures of solid state polymorphs of sulfur trioxide contains tetrahedral SO4 units (a) Y-SO3 consists of trimeric units and (b) a- and P-SO3 contain polymeric chains. Colour code S, yellow O, red. 

The third law of thermodynamics can be verified experimentally. The stable rhombic low-temperature modification of sulfur transforms to monoclinic sulfur at 368.5 K (p = 1 bar). At that temperature, Ttrs, the two polymorphs are in equilibrium and the standard molar Gibbs energies of the two modifications are equal. We therefore have... 

Three of the polymorphs of cyclooctasulfur are firmly established orthorhombic a-sulfur, monoclinic j8-sulfur, and monoclinic y-sulfur. Their structure, along different axes, is shown in Fig. 6. Over a dozen of... 

Systematic studies of the structural properties of TaS2 by Jellinek10 revealed the presence of several polymorphic forms of the compound as a function of temperature. Within a layer, the tantalum atom sits in the holes formed between two layers of sulfur atoms in the S—Ta—S sandwich. The coordination of the tantalum is trigonal prismatic or octahedral, depending on whether the two sulfur layers lie one on top of the other or are rotated by 60°. Thus the various poly-... 

XRD analysis has revealed the presence of several different types of carbonate minerals in liquefaction residues from a number of coals. Minerals identified included vaterite and calcite (two polymorphs of CaCO,) dolomite (CaMg[C03]2) and in the residue from a high sulfur coal (2.26 db), anhydrite (CaSO,) was identified. The types of mineral deposits formed depend not only on the coal but also on the reaction conditions. Our data indicates that whilst vaterite forms at low temperatures (380°C), as the temperature increases, the vaterite becomes progressively converted to calcite, the more stable form. After further increases in temperature, particularly at long reaction times, dolomite begins to form. 

Cramer, F., Sprinzl, R., Furgac, N Freist, W., Saenger, W., Manor, P, Sprinzll, L. and Stemback, H. (1974). Crystallization of yeast phenylalanine transfer RNA polymorphism and smdies of sulfur-substimted mercury binding derivatives. Biochim. Biophys. Acta, 349, 351-65. [19]... 

Both the low (B) and high (o) temperature polymorphs of NiS dissolve excess sulfur to form solid solutions. The B phase shows a limited range of homogeneity but the a phase shows a broad limit extending to approximately NiS (8). The o phase is normally designated o-Ni to emphasize this feature. However, the sulfur-poor limit of this phase has the stoichiometric composition to at least 873 K at 1070 K the deviation from stoichiometry is less than 0.05 weight percent nickel (8). Above 1079 K the deviation of the sulfur-poor limit from stoichiometry increases more rapidly and a material of stoichiometry NiS will exist as an equilibrium mixture of liquid NiyS. a hd solid a-Nij S (8). Thus, NiS melts incongruently. 

The melting curve of sulfur with respect to metastability and experimental procedures has been critically analyzed by Vezzoli and Walsh [194]. The authors also discussed the previously reported phase boundaries of the sohd high-pressure allotropes in the range up to 4 GPa and from room temperature up to about 670 K [132]. However, the structures of these high-pressure polymorphs are unknown and their probability to exist under equilibrium conditions is still awaiting confirmation. 

The X-ray measurements of Robertson and his co-workers were in all cases carried out on the more stable /3 modification, which is also the more crystalline modification. The a modification is unstable to transition to the 8 form in aromatic solvents (335). Ebert and Gottlieb have published (81) detailed methods by which the different polymorphs of copper, nickel, zinc, and metal-free phthalocyanine may be isolated. Phthalonitrile condenses in hydrogen-donor solvents, such as cyclohexylamine, to form metal-free phthalocyanine. The a metal-free phthalocyanine may be obtained from this reaction by milling the product in the dry with sodium chloride, the 6 form by milling in the presence of xylene. Solutions of metal-free phthalocyanine in sulfuric acid precipitate the a modification upon dilution (172). This may be sublimed below 200°C in vacuo, but is converted to the j8 modification above this temperature. Copper phthalocyanine behaves similarly. The 6 phthalocyanine may be reconverted to the a form by milling in the dry with sodium chloride or sodium sulfate. The y metal-... 

Wurtzite is a second polymorph of ZnS in contrast to the cubic symmetry of zinc blende, wurtzite has hexagonal symmetry. In the three unit cells shown in Figure 5.20, the 12 ions in corner sites define a hexagonal prism. Each of the zinc and sulfur centres is tetrahedrally sited, and a unit cell in which Zn and S are interchanged with respect to Figure 5.20 is equally vahd. 

Selenium and tellurium have the A8 helicoidal zigzag chain structure at ambient conditions (1). Both show maxima in their melting curves at about 50 kb at 963°K for selenium (32) and at an average p-T of about 12 kb at 743°K (22, 29, 33, 34, 35) for tellurium. These maxima are suspected to be related to the maximum in the melting curve of sulfur at 86 kb at 953°K (12) because at these conditions the sulfur polymorph... 

Very complicated phase diagrams can arise when substances can exist in more than two crystalline polymorphs. In certain cases, some of the forms may be enantiotropic to each other, and monotropic to yet others. For instance, of the eight polymorphs of elemental sulfur, only the monoclinic and rhombic modifications exhibit enantiotropy and the possibility of reversible interconversion. All of the other forms are monotropic with respect to the monoclinic and rhombic forms and remain as metastable phases up to the melting point. 

For solids capable of exhibiting polymorphism, in the vicinity of the Sj-Sj-V triple point, the sublimation curve for the metastable phase (Sj-V) will always lie above the sublimation curve for the stable phase (S,-V). It follows that the vapor pressure of a metastable solid phase will always exceed the vapor pressure of the stable phase at a given temperature. This generalization was first deduced by Ostwald, who proved that for a given temperature of a one-component system, the vapor pressure of any metastable phase must exceed that of the stable phase [25]. This behavior was verified for the rhombic and monoclinic polymorphs of elemental sulfur, where it was found that the ordinary transition point of the enantiotropic conversion was 95.5°C [26]. The vapor pressure curve of the rhombic phase was found invariably to exceed that of the monoclinic phase at all temperature values above 95.5°C, while the vapor pressure of the monoclinic phase was higher than that of the rhombic phase below 95.5°C. This behavior provided direct evidence that the rhombic phase was the most stable... 

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