Boron and Borides

The modifications of boron and boron compounds are characterized by nets or chains of boron atoms bound to each other by covalent bonds. The coordination number of the boron atoms varies between 1 and 7. The nets have the form of linked icosahedra, cuboctahedra, octahedra, graphite-like planes, or linear branched boron chains. Two-electron three-center bonds in boron-cornered triangles are the rule in the lattice structure. Boron can only form ions in combination with hydrogen (BH4 ) and oxygen (BOj ). The borates (boron oxide derivatives) form covalent nets like the silicates. They are often amorphous. 

---

K. E. Spear, in Proc. Int. Symp., Boron and Borides, Tbilisi, USSR, 1972. 

There have been several reviews written on boron and borides, which include various reports on the well-known rare earth borides, such as RB2, RB4, RBe, RB12, and RBee which were first synthesized over a half century ago. The novel higher borides discovered in the past decade with [B]/[R] = n, with n > 12 have been included in a review. 

In this Chapter, we describe mainly the investigations from our laboratory directed toward the synthesis of stoichiometric boron-based nanostructures. Our goal has been to create new boron and boride nanostructures with advanced properties and desired dimensionality. Through a detailed investigation on the structures and properties of new boron-based nanomaterials, we... 

AC Switendick. The electronic structure of boron and borides The evolution from cubic to rhombo-hedral based structures. In H Werheit, ed. Proceedings of the 9th International Symposium on Boron, Borides and Related Compounds, Duisburg. University of Duisburg, 1987, p 349. 

Boron forms B—N compounds that are isoelectronic with graphite (see Boron compounds, refractoryboron compounds). The small size also has a significant role in the interstitial alloy-type metal borides boron forms. Boron forms borides with metals that are less electronegative than itself including titanium, zirconium, and hafnium. 

Table 1 fists many metal borides and their observed melting points. Most metals form mote than one boride phase and borides often form a continuous series of solid solutions with one another at elevated temperatures thus close composition control is necessary to achieve particular properties. The relatively small size of boron atoms facilitates diffusion. 

The main chemical products produced from these minerals are (a) boron oxides, boric acid and borates, (b) esters of boric acid, (c) refractory boron compounds (borides, eu .), (d) boron halides, (e) boranes and carbaboranes and (f) organoboranes. The main industrial and domestic uses of boron compounds in Europe (USA in parentheses) are ... 

V. I. Matkovich (cd.). Boron and Refractory Borides, Springer-Verlag, Berlin, 1977, 656 pp. 

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

The structures of boron-rich borides (e.g. MB4, MBfi, MBio, MB12, MBe6) are even more effectively dominated by inter-B bonding, and the structures comprise three-dimensional networks of B atoms and clusters in which the metal atoms occupy specific voids or otherwise vacant sites. The structures are often exceedingly complicated (for the reasons given in Section 6.2.2) for example, the cubic unit cell of YB e has ao 2344 pm and contains 1584 B and 24 Y atoms the basic structural unit is the 13-icosahedron unit of 156 B atoms found in -rhombohedral B (p. 142) there are 8 such units (1248 B) in the unit cell and the remaining 336 B atoms are statistically distributed in channels formed by the packing of the 13-icosahedron units. 

Although boron forms borides with many elements, only the borides of the transition metals have been investigated extensively for their CVD characteristics. Boron forms stable borides with the transition metals, and the most refractory of these and those with the greatest potential interest are the borides of the elements of Groups IVa (Ti, Zr, Hf), Va (V, Nb, Ta) and, to a lesser degree. Via (Cr, Mo, W) (see Table... 

Unlike bonding, direct boride deposition does not require a reaction with the substrate to form the boride. Both boron and metal atoms are supplied as gaseous compounds. 

---