Icosahedral boron carbides

GA Samara, HL Tardy, EL Venturini, TL Aselage, D Emin. Low temperature dielectric properties and AC conductivities of the icosahedral boron carbides. In D Emin, T Aselage, AC Switendick, B Morosin, CL Beckel, eds. Boron-Rich Solids, AIP Conference Proceedings 231. Albuquerque ... 

Theoretically, since these are layered homologous compounds, a numer-ous/infinite number of compounds are possible in the family However, realistically, we have been able to synthesize pure phases of only the three compounds. Compounds which contained more than four layers of the B12 icosahedral and C-B-C chain layers (which is the case for RB28.5C4) always contained a mixture of other number layers also. In the limit of the boron icosahedra and C-B-C chain layers separating the metal layers reaching infinity (i.e. no rare earth layers) the compound is actually analogous to boron carbide. In the opposite limit, a compound with just one boron icosahedra layer is imaginable. And in actuality, such a MgB9N compound was independently discovered by Mironov et al. (2002). However, such a compound with rare earth atoms has not yet been synthesized. 

The reader should note the similarity between the icosahedral fragment in BioHh and the icosahedron that is present not only in crystalline elemental boron, but also in a number of boron compounds such as boron carbide (which may be represented as B12C3), boron suboxide (B12BO2), and the salt K2B12H12. Moreover, similar icosahedra of aluminum atoms... 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

The B mas and static NMR spectra of a series of boron carbides show a broad major resonance at about 1.3 to - 4.6 ppm, the peak position varying almost linearly with carbon content (Figure 7.10A). This resonance has been assigned to boron in the B-rich icosahedral units which are bonded together both directly and via three-atom chains (Kirkpatrick etal. 1991). A small additional shoulder on the major resonance of the static B spectra (Figure 7.10B) which increases in intensity with decreasing C content and can be simulated as a second-order quadrupolar lineshape has been assigned to the boron site in the centre of the various possible C-B-C chains (Kirkpatrick et al. 1991). 

H. C. Longuet-Higgins and M. de V. Roberts, who predicted thereby that the icosahedron of 12 boron atoms familiar from elemental boron, boron carbide, and some borides should be stabilized in molecular hydride form, not as the neutral entity B Hu (which if icosahedral would be a diradical) but as the dianion [B12H12] , which contained the 25 valence shell electron pairs needed for the 12 exo B-H bonds and 13 skeletal bonding MOs. Subsequent MO treatments of the closo deltahedral anions B I 1 and carboranes (AB, 2H, in Figure 3.1 have shown that these are the shapes that make best bonding use of their (n + 1) pairs of electrons available for skeletal bonding. ... 

Controlled pyrolysis of a BBr3-CH4 H2 mixture over a BN surface at 1550—1650 C produces a rhombohedral boron carbide B13C2. This is believed to contain icosahedral B12 units and linear CBC chains. 

Carbides Boron Carbide (B4C) isomorphous with TiB2 a=3.1694-3.170A c=3.528-3.5365A c/a=1.114 rhombic, C3 chains and B12 icosahedral in a NaCl structure, extended along a body diagonal... 

Lazzari, R. VasL N. Besson, J. M. Baroni, S. and Dal Corso, A. Atomic structure and vibrational properties of icosahedral B4C boron carbide. Phys. Rev. Lett., 1999, 83, 3230-3233. 

Table 8 shows that, for a-rhombohedral boron, p-rhombohedral boron and boron carbide, the electron deficiencies determined by electronic band structure calculations are correlated with the densities of defects in the structures. Since calculations of the electronic properties of defects in icosahedral boron-rich solids are missing, the following assumptions (202) were made in accordance with general results obtained on defects in semiconductors. 

GH Kwei, B Morosin. Structure of the Boron-Rich Boron Carbides from Neutron Powder Diffraction Implication for the Nature of the Inter-Icosahedral Chains. J Phys Chem 100 8031, 1996. 

H Werheit. Optical properties of boron-rich solids with icosahedral structure. In R Freer, ed. The Physics and Chemistry of Carbides, Nitrides and Borides. Dordrecht Kluwer, 1990, p 705. Gmehn Handbook of Inorganic Chemistry. Vol 54. Boron Compounds. Part 20. Berlin Springer, p 224 (1979). 

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