Rhombohedral

The first complete description of crystalline Se was reported by Donohue et al. in 1961 on the basis of an X-ray diffraction study [64]. The rhombohedral structure was verified, and the molecular symmetry was ascertained to be D d. Since cyc/o-hexasulfur decomposes rapidly under X-ray irradiation at standard temperature-pressure (STP) conditions Steidel et al. reinvestigated the molecular and crystal structure at 183 K giving results with a higher accuracy [65]. The molecular and crystal lattice parameters are summarized in Tables 1 and 2. 

As listed in Table 2 and shown in Fig. 1, each of the Ss molecules has 18 short intermolecular contacts in the range 350-353 pm (at 300 K). This fact, in combination with the compact molecular structure, accounts for the high density of rhombohedral 85 (for comparison see the structure data of orthorhombic sulfur. Table 6). On the other hand, the compact molecular structure is responsible for a certain strain in the bond geometry which is expressed by a relatively large deviation of the torsion angle from an unstrained value of about 90°, therefore, making the molecule unstable [66]. 

Since the lattice parameters depend significantly on the temperature (Table 2), it is possible to estimate the coefficient of isobaric thermal expansion roughly to about 2.8x10 K  

Up to now no six-membered sulfur allotrope other than rhombohedral 85 has been found. In addition, from theoretical structure analysis it is reasonable to assume that the chair conformation is energetically more favorable than 

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Figure Cl.2.8. Ortliorhombic ID and rhombohedral 2D stmcture of polymerized [60]fullerene.

Graphite exists in two forms alpha and beta. These have identical physical properties, except for their crystal structure. Naturally occurring graphites are reported to contain as much as 30% of the rhombohedral (beta) form, whereas synthetic materials contain only the alpha form. The hexagonal alpha type can be converted to the beta by mechanical treatment, and the beta form reverts to the alpha on heating it above lOOOoC. 

Triclinic Rhombohedral Three unequal axes intersecting obliquely Two equal axes making equal angle with each other No planes or axes of symmetry a b c y 90°... 

Texture. All limestones are crystalline, but there is tremendous variance in the size, uniformity, and arrangement of their crystal lattices. The crystals of the minerals calcite, magnesite, and dolomite are rhombohedral those of aragonite are orthorhombic. The crystals of chalk and of most quick and hydrated limes are so minute that these products appear amorphous, but high powered microscopy proves them to be cryptocrystalline. Hydrated lime is invariably a white, fluffy powder of micrometer and submicrometer particle size. Commercial quicklime is used in lump, pebble, ground, and pulverized forms. 

Several aHotropes of black phosphoms have also been reported (2). These include one amorphous and three crystalline modifications. At increasing pressures and temperatures reaching above 1200 MPa (12 kbar) and several hundred degrees, a series of black phosphoms modifications are formed that are characterized by even higher densities (2.70 g/cm ). These include orthorhombic, rhombohedral, and cubic varieties. The black forms have lower reactivity and solubiUty than red phosphoms. 

The reaction conditions are critical, as hydrated iron oxide, Fe202 H20, can also precipitate. The particles are either spherical or rhombohedral, depending on the nucleating material. 

Silver nitrate forms colorless, rhombic crystals. It is dimorphic and changes to the hexagonal rhombohedral form at 159.8°C. It melts at 212°C to a yellowish Hquid which solidifies to a white, crystalline mass on cooling. An alchemical name, lunar caustic, is stiU appHed to this fused salt. In the presence of a trace of nitric acid, silver nitrate is stable to 350°C. It decomposes at 440°C to metallic silver, nitrogen, and nitrogen oxides. Solutions of silver nitrate are usually acidic, having a pH of 3.6—4.6. Silver nitrate is soluble in ethanol and acetone. 

Antimony(III) iodide [7790-44-5] Sbl, forms red rhombohedral crystals, intermediate in stmcture between a molecular and an ionic crystal. In Sbl vapor there is no indication of association. 

The physical properties of elemental boron are significantly affected by purity and crystal form. In addition to being an amorphous powder, boron has four crystalline forms a-rhombohedral, P-rhombohedral, a-tetragonal, and -tetragonal. The a-rhombohedral form has mp 2180°C, sublimes at approximately 3650°C, and has a density of 2.45 g/mL. Amorphous boron, by comparison, has mp 2300°C, sublimes at approximately 2550°C, and has a density of 2.35 g/mL. 

Boron is an extremely hard refractory soHd having a hardness of 9.3 on Mohs scale and a very low (1.5 x 10 ohm cm ) room temperature electrical conductivity so that boron is classified as a metalloid or semiconductor. These values are for the a-rhombohedral form. 

The a-rhombohedral form of boron has the simplest crystal stmcture with slightly deformed cubic close packing. At 1200°C a-rhombohedral boron degrades, and at 1500°C converts to P-rhombohedral boron, which is the most thermodynamically stable form. The unit cell has 104 boron atoms, a central B 2 icosahedron, and 12 pentagonal pyramids of boron atom directed outward. Twenty additional boron atoms complete a complex coordination (2). 

Lower Oxides. A number of hard, refractory suboxides have been prepared either as by-products of elemental boron production (1) or by the reaction of boron and boric acid at high temperatures and pressures (39). It appears that the various oxides represented as B O, B O, B22O2, and B23O2 may all be the same material ia varying degrees of purity. A representative crystalline substance was determined to be rhombohedral boron suboxide, B12O2, usually mixed with traces of boron or B2O3 (39). A study has been made of the mechanical properties of this material, which exhibits a hardness... 

Properties. Boron carbide has a rhombohedral stmcture consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms ia a linear chain outside the icosahedra (3,4,6,7). Thus a descriptive chemical formula would be [12075-36-4]. 

Each boron atom is bonded to five others ia the icosahedron as well as either to a carbon atom or to a boron atom ia an adjacent icosahedron. The stmcture is similar to that of rhombohedral boron (see Boron, elemental). The theoretical density for B22C2 is 2.52 g/mL. The rigid framework of... 

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