Black crystalline /3-rhombohedral

Boron (B), the second hardest element, is the only allotropic element in Group 13. It is second only to carbon (C) in its ability to form element-element bonded networks. Thus, in addition to amorphous boron, several different allotropes of boron are known, of which three are well characterized. These are red crystalline a-rhombohedral boron, black crystalline /3-rhombohedral boron (the most thermodynamically stable allotrope), and black crystalline /3-tetragonal boron. All are polymeric and are based on various modes of condensation of the Bj2 icosahedron (Figure 2). 

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 determination of precise physical properties for elemental boron is bedevilled by the twin difficulties of complex polymorphism and contamination by irremovable impurities. Boron is an extremely hard refractory solid of high mp, low density and very low electrical conductivity. Crystalline forms are dark red in transmitted light and powdered forms are black. The most stable ()3-rhombohedral) modification has mp 2092°C (exceeded only by C among the non-metals), bp 4000°C, d 2.35 gcm (a-rhombohedral form 2.45gcm ), A77sublimation 570kJ per mol of B, electrical conductivity at room temperature 1.5 x 10 ohm cm- . 

A review of the alleged allotropes of phosphorus reduces their number to four, namely, the a- and/3-forms of yellow phosphorus, red or violet phosphorus, and black phosphorus. Most of the work of various investigators has been directed towards elucidating the nature of red phosphorus, and of the transformation of yellow to red phosphorus and conversely. Red phosphorus was formerly considered to be amorphous, and it was often called amorphous phosphorus. The term amorphous, however, here referred more to the general appearance of the powder rather than to its minute structure. J. W. Retgers 5 showed that the particles of ordinary red phosphorus are rhombohedral crystals, which are well developed in those of W. Hittorf s violet phosphorus. All four varieties are therefore crystalline. J. W. Terwen has reviewed this subject in a general way and M. Copisarow discussed the theory of allotropy,... 

Black phosphorus is thermodynamically the most stable form of the element and exists in three known crystalline modifications orthorhombic, rhombohedral, and cubic, as well as in an amorphous form. Unlike white phosphorus, the black forms are all highly polymeric, insoluble, and practically non-flammable, and have comparatively low vapor pressures. The black phosphorus varieties represent the densest and chemically the least reactive of all known forms of the element. 

The most thermodynamically stable, and least reactive, form of phosphorus is black phosphorus, which exists as three crystalline (orthorhombic-, rhombohedral- and metallic, or cubic, and one amorphous, allotrope. All are polymeric solids and are practically nonflammable. Both orthorhombic and rhombohedral phosphorus appear black and graphitic, consistent with their layered structures. 

Black Phosphorus. Polymorphic. Orthorhombic crystalline form stable in air resembles graphite in texture produced from the white modification under high pressures Bridgman, J. Am. Chem. Soc. 36, 1344 (1914) Jacobs, J. Chem. Phys. 5, 945 (1937) Krebe, Inorg. Syn. 7, 60 (1963), d 2.691. Does not catch fire spontaneously. Insol in organic solvents. Amorphous form prepd at lower pressures Ja-cobe, loc. cit At higher pressure the orthorhombic form undergoes reversible transition to a rhombohedral structure, d 3.56, and a cnbic structure, d 3.83 Jamieson, Science 139, 1291 (1963). 

The form of elemental antimony that is stable at normal temperatures and pressures is the gray, metallic rhombohedral a-form, mp 630.7 °C, bp 1587 °C, density 6.70 gcm. Crystals are lustrous. They have a relatively high electrical resistivity (41.7 1 2cm at 20°C). The structure of o -8b consists of sheets of covalently bonded antimony stacked in layers, which are formed of puckered slx-membered rings. Each antimony forms three shorter bonds (2.91 A) in the same layer as well as three longer bonds (3.36 A) to antimony atoms in the adjacent layer. In addition to the a-form, other allotropes include a very unstable yellow form and black forms obtained electrolytically or by condensing the vapor on cold surfaces. Two crystalline allotropes are made by high-pressure techniques. At 85 kbar, a modification with a primitive cubic lattice is formed where each antimony atom is in an octahedral environment of six equidistant (2.96 A) neighbors. Further... 

It is a reddish, odorless, tasteless solid, which does not fume in air, nor dissolve in the solvents of the yellow variety. Sp. gr. 2.1. Heated to 500° (932° P.) with lead, imthe absence of air, it dissolves in the molten metal, from Avhich it separates on cooling in violet-black, rhombohedral crystals, of sp. gr. 2.34. If prepared at 250° (482° P.) it fuses below that temperature, and at 260° (.500° P.) is transformed into the yellow variety, which distils. The crystalline product does not fuse. It is not luminous at ordinary temperatures. 

The hexagonal NbSg and the rhombohedral Nbj+xSg are blue-black, shiny crystalline compounds NbSg single crystals 0.5 mm. in size can be obtained. The remaining niobium sulfides are dark-gray to black or dark-brown powders devoid of luster. 

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