P-rhombohedral boron

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). 

S.3. Crystallographic Data on Solid Solutions of p-Rhombohedral Boron. 

Fig. 12.6 The construction of the Bg4-unit, the main building block of the infinite lattice of P-rhombohedral boron.

Chemical element B has two stable isotopes, B and "B, nuclei of which differ by one neutron. In the universe, "B/ B natural abundance ratio is estimated as 4.05 0.10 (Viola 1991). This value coincides with the natural isotopic composition of boron, 19.85% B+80.15% "B, determined in floating-zone-melted P-rhombohedral boron by the glow-discharge mass-spectrometry laboratory method (Nogi et al. 2000). Since boron and its compounds have found a variety of applications, the elemental and isotopic analyses of boron become important steps of an investigation with special challenges (Balaram 2011). 

Structures of powdered P-rhombohedral boron and amorphous boron were investigated with pulsed neutron diffraction techniques (Delaplane et al. 1988). To avoid intensive neutron absorption by °B nuclei, samples were "B isotopically enriched up to 97.1% and 99.1%, respectively. Earlier neutron diffraction studies based on nuclear reactor data did not permit the derivation of a meaningful radial distribution of atoms in amorphous material due to limited range of the neutron wave vector (<10.8 A" ). The obtained static structural factor and derived radial distribution function supported a structural model of amorphous boron based on building blocks of B,2 icosahedra resembling those found in p-rhombohedral boron, but with disorder occupying in the linking between ico-sahedral subunits. The intensity data indicated that amorphous samples contained 5% of a mixture of crystalline a- and p-rhombohedral boron. 

Jemmis, E. D. and D. L. V. K. Prasad. 2006. Icosahedral B12 macropolyhedral boranes, P-rhombohedral boron and boron-rich solids. J. Solid State Chem. 179 2768-2774. 

Although the number of allotropes known for elemental boron is 16, the structure of most of them remains ambiguous, a- rhombohedral boron and p-rhombohedral boron are the two allotropic forms with unambiguous crystal structure and electronic structure [41]. The unit cell ofa-boron is a Bj2 unit (Figure 5.31). Each boron atom can provide one electron for exohedral bonding. The red-labeled boron atoms... 

M.M., and Jemmis, E.D. (2005) Electronic structure and bonding of P-rhombohedral boron using cluster fragment approach. Phys. Rev. B, 72 (19), 195102. 

Kobayashi, M. et al. (1995) Rietveld analysis of LiB13 with p-rhombohedral boron structure. J. Alloys Compd., 221 (1-2), 120-124. 

Callmer, B. and Lundstrom, T. (1976) A single-crystal diffractometry investigation of iron in p-rhombohedral boron. J. 

Kuhlmann, U. et al. (1992) Influence of interstitially soluted iron on stmc-tural, optical and electrical properties of P-rhombohedral boron. J. Alloys Compd., 186 (2), 187-200. 

VM Fig. 13.6 The construction of the Bg -unit, the main building block of the infinite lattice of P-rhombohedral boron, (a) In the centre of the unit is a B] 2-icosahedron, and (b) to each of these twelve, another boron atom is covalently bonded, (e) A B o-cage is the outer skin of the Bg4-unit. (d) The final Bg -unit can be described in terms of covalently bonded sub-units (Bi2)(Bi2)(Bgo). 

P-Rhombohedral boron The p-rhombohedral variety of boron, also called the high-temperature form, is prepared either by reduction of a boron halide with hydrogen on a substrate heated to a temperature above approximately 1400° C, or by solidification of liquid boron, or by annealing (a-rh.) boron beyond the (a-rh.) - (p-rh.) transformation point. That means, the a-rhombohedral boron ° transforms by 1200° C into the more stable solid p-boron. The p-rh. boron is composed of much complicated Bi2(B6)i2 polyhedra. " The structure of p-boron consists of B12 icosahedra and B28 subunits which can be regarded as triplex icosahedra. The lattice constants of p-rhombohedral boron are (a =10.145 A, a = 65° 17 space... 

Boron. - IR spectra were reported for natural, B- and B-enriched P-rhombohedral boron single crystals. ... 

The essential bases for the structural diversity within this structure group are different holes in the boron framework (Fig. 2), which are large enough to accommodate foreign atoms up to certain solubility limits, which seem to be specifically determined by the number of suitable sites, the size of the foreign atoms, or the degree of electron transfer to the boron framework (see later). Numerous binary and ternary compounds of this type with Mg, Al, Ga, Si, Ge, Cu, Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, Fe, Co, and Ni atoms have been prepared (e.g., see Ref. 27 and references therein) for LiBis with the P-rhombohedral boron structure see (28). 

Amorphous boron consists of B12 icosahedra, which are statistically distributed. It has been proved that the narrow-range and the medium-range orders of amorphous boron are closely related to those of crystalline P-rhombohedral boron (34-38). Accordingly, it has been proved that the external bonds of the icosahedra in amorphous boron are largely covalently saturated. Therefore it seems a likely supposition that the electronic properties are closely related to those of P-rhombohedral boron as well, and moreover there may be holes in the structure to accommodate foreign atoms for doping to modify the properties. This would give a favorable chance of future application because well-established techniques could be used to prepare amorphous instead of crystalline material, for example, in thin layers. 

Because the solubility coefficients of carbon in the solid and the liquid phase are almost the same, zone melting, which is used to prepare high-purity crystals of many other elements, is not suitable in the case of boron. Technical boron, which is often taken as the ingredient for the preparation of boron compounds, contains up to about 0.5% carbon. However, in several preparative methods for boron compounds the carbon content may be reduced by secondary chemical or physical reactions. The purest P-rhombohedral boron crystals that have become available up to now were produced by Wacker-Chemie, Munich, FRG. Despite the claimed purity of 99.9999% with respect to other elements, even this high-purity boron contains carbon in concentrations of typically 30 to 80 ppm. Therefore, apart from boron carbide containing carbon as a determining bonding partner, in the assessment of the properties of boron and boron compounds attention must be paid to the fact that a certain, usually unknown carbon content could have influenced the properties determined. 

Quantitative investigations of the effect of the carbon content on the structure and physical properties of boron-rich solids have been restricted to p-rhombohedral boron and boron carbide (56-61). Up to carbon contents of about 1 at.% in p-rhombohedral boron, the carbon atoms substitute statistically for boron atoms at the polar sites of the Bjj icosahedra in the structure with a maximum of one carbon atom per icosahedron. Compared with the boron atom, the... 

For the influence of the carbon content on the physical properties of P-rhombohedral boron and boron carbide, see later. 

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