Boron polymorphs

The tertiary metal phosphates are of the general formula MPO where M is B, Al, Ga, Fe, Mn, etc. The metal—oxygen bonds of these materials have considerable covalent character. The anhydrous salts are continuous three-dimensional networks analogous to the various polymorphic forms of siHca. Of limited commercial interest are the alurninum, boron, and iron phosphates. Boron phosphate [13308-51 -5] BPO, is produced by heating the reaction product of boric acid and phosphoric acid or by a dding H BO to H PO at room temperature, foUowed by crystallization from a solution containing >48% P205- Boron phosphate has limited use as a catalyst support, in ceramics, and in refractories. 

The thermodynamically most stable polymorph of boron is the /3-rhombohedral modification which has a much more complex structure with 105 B atoms in the unit cell (no 1014.5 pm, a 65.28°). The basic unit can be thought of as a central Bn icosahedron surrounded by an icosahedron of icosahedra this can be visualized as 12 of the B7 units in Fig. 6.1b arranged so that the apex atoms form the central Bn surrounded by 12 radially disposed pentagonal dishes to give the Bg4 unit shown in Fig. 6.3a. The 12 half-icosahedra are then completed by means of 2 complicated Bjo subunits per unit cell,... 

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- . 

N. Dubrovinskaia, V. L. Solozhenko, N. Miyajima, V. Dmitriev, O. O. Kurakevych, and L.Dubrovinsky, Superhard Nanocomposite of Dense Polymorphs of Boron Nitride Noncarbon Material has Reached Diamond Hardness , Appl. Phys. Lett., 90,101912 (2007). 

Boron and nitrogen can form the following BN compounds which are iso-structural polymorphs of carbon. 

Elemental boron can have different crystalline polymorphs, depending on the conditions of deposition. On crystallization from the melt or chemical vapor deposition above 1300°C, one gets j8-rhombohedral. At temperatures below 1300°C, the most commonly observed structure is a-rhombohedral. 

Binary borides are formed with transition and inner transition elements, with aUcah and alkaline-earth elements, and with the p elements, aluminnm, and sihcon. In addition, a gronp of more electronegative elements (C, O, P, S, As, Se) form componnds with boron, which display properties and stmctures closely related to those of some borides. This group of compounds is also briefly described in the present article as well as the polymorphs of elemental boron. 

J. L. Hoard and R. E. Hughes, Elemental Boron and Compoimds of High Boron Content Structure, Properties and Polymorphism, in The Chemistry of Boron and Its Compoimds , ed. E. L. Muetterties, Wiley, New York, 1967, p. 25. 

Boron nitride (BN) is a versatile ceramic that is found and used in various crystallographic and geometrical forms. Although there are chemical similarities, the conditions for the synthesis and fabrication of each form (hexagonal, cubic, polymorphs of BN, fibers, two-dimensional forms and composites) have different requirements and are... 

A feature of the polymorphs of boron is the large proportion of atoms which form a sixth bond (pentagonal pyramidal coordination) directed outwards along the 5-fold axis of the icosahedron. The fractions are ... 

Fig. 12.18 Part of the layer structure of the common polymorph of boron nitride, BN. Hexagonal rings in adjacent layers lie over one another so that B and N atoms are eclipsed. This is emphasized by the yellow lines.

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