Allotropes boron

The a tetragonal form, firstly reported as a boron allotrope, has been reformulated as B50C2 or B50N2, depending on the preparative conditions it never forms in absence of carbon or nitrogen. 

The /3-R105 boron allotrope has a much more complex structure with 105 B atoms in the unit cell (space group R3m, a = 1014.5 pm, a = 65.28°). A basic building unit in the crystal structure is the B84 cluster illustrated in Fig. 13.2.5(a) it can be considered as a central Bi2a icosahedron linked radially to 12 B6 half-icosahedra (or pentagonal pyramids), each attached like an inverted umbrella to an icosahedral vertex, as shown in Fig. 13.2.5(b). 

C. Boron Allotropes The Truncated Icosahedron in a Boron Structure... 

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

This section will focus on homonuclear neutral or anionic clusters of the elements aluminum, gallium, indium, and thallium, which have an equal number of cluster atoms and substituents. Thus, they may clearly be distinguished from the metalloid clusters described below, which in some cases have structures closely related to the allotropes of the elements and in which the number of the cluster atoms exceeds the number of substituents. The compounds described here possess only a single non-centered shell of metal atoms. With few exceptions, their structures resemble those of the well-known deltahedral boron compounds such as B4(CMe3)4 [30], B9CI9 [31] or [B H ]2 [32]. The oxidation numbers of the elements in these... 

A typical building block used to construct several solid-state structures (boron-rich borides and allotropes of elemental boron) is the B12 icosahedron. According to King, (1993) an icosahedral B12 building block in which each of the 12 vertices contributes a single electron for an external two-electron two-centre (2e, 2c) bond to an external group implies the following electron count ... 

Several allotropic forms of boron are known which are based on various ways of joining B12 icosahedra (using the external orbitals on each boron atom). 

Elementary boron exists in the solid state in several allotropic forms. Those which have been properly characterized by X-ray diffraction contain icosahedral B12 units, which are also the basis of many boron cluster compounds. 

The electron-deficient character of boron also affects its allotropic forms. The high ionization energies and small size prevent boron from adopting... 

The crystal structures of boron are complex. As many as 16 distinct allotropes have been reported, but some have been poorly characterized. Eight of these have been studied as single crystals and others as... 

The a-rhombohedral form of boron (a-boron) was first reported by L. V. McCarty, J. S. Kasper, F. H. Horn, B. F. Decker, and A. E. Newkirk.1 Of the many allotropic forms of boron, it has the simplest structure.2 It may be prepared by the pyrolysis of boron(III) iodide on a tantalum filament at 800-1000°C., but the product is usually contaminated by other allotropic varieties of boron.1,4 Recently, Hagenmuller and Naslain showed that boron(III) bromide may be reduced by... 

The uniqueness of boron is clearly seen in its elemental forms, the number and structural complexity of which exceed those of any other element. At least five distinct allotropes are known, all of which contain icosahedral B12 cluster units that in most cases are accompanied by other boron atoms lying outside the icosahedral cages. The most thermodynamically stable form, j8-rhombohedral boron, has 105 B atoms in its unit cell, while the /3-tetragonal phase has 192 atoms and is still not completely elucidated despite years of study ... 

Elements along a rough diagonal from boron to polonium are intermediate in behavior, in some cases having both metallic and nonmetallic allotropes (elemental forms) these elements are designated as metalloids or semimetals. As described in Chapter 7, some elements, such as silicon and germanium, are capable of having their conductivity finely tuned by the addition of small amounts of impurities and are consequently of enormous importance in the manufacture of semiconductors in the computer industry. 

Boron s chemistry is so different from that of the other elements in this group that it deserves separate discussion. Chemically, boron is a nonmetal in its tendency to form covalent bonds, it shares more similarities with carbon and silicon than with aluminum and the other Group 13 elements. Like carbon, boron forms many hydrides like silicon, it forms oxygen-containing minerals with complex structures (borates). Compounds of boron have been used since ancient times in the preparation of glazes and borosilicate glasses, but the element itself has proven extremely difficult to purify. The pure element has a wide diversity of allotropes (different forms of the pure element), many of which are based on the icosahedral Bj2 unit. 

One of the unusual properties of boron is the many physical forms, called allotropes, in which it occurs. Allotropes are forms of an element with different physical and chemical properties. One form of boron consists of clear red crystals with a density of 2.46 grams per cubic centimeter. A second form consists of black crystals with a metallic appearance and a density of 2.31 grams per cubic centimeter. Boron can also occur as a brown powder with no crystalline stmcmre. The density of this powder is 2.350 grams per cubic centimeter. 

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