Unit borides

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... 

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. 

Among metal borides of the formula MjM B or (Mj, M/r)2B, the competing structural units are (a) the antiprism and (b) the trigonal metal prism. In many cases the CUAI2 structure with BMg-antiprismatic B coordination is adopted in close resemblance to transition-metal silicides, but no boron-carbon substitution is ob-served - " . 

Formation of tetrahedral My4 clusters is the structural unit common to all known MRgMT4B4-type phases with strong M.,—B, but weak B-M g interaction in accord to the characterization of borides. Involving two remote Mre atoms, the B... 

The cubic UB, 2-type boride structure with space group Fm3m can be described on the basis of a B,2-cubooctahedron (see Fig. 1) . The association of the B,2-poly-hedra by oriented B—B bonds gives rise to a three-dimensional skeleton with boron cages. Formally, the arrangement of the B,2-units and of the metals atoms is of the NaCl-type. Each metal is located in the center of a B24-cubooctahedron. 

Nonclassical type structures of borides involving different basic units of B atoms, e.g., the B5 pentagonal pyramid in MgB4, the B octahedron in Sm2B3 and finally the B 2 octahedra in Mg2Bi4, MgAlBi4 and NaxBogBi4, are considered here. 

The distribution of the observed higher borides among the five structural types (MB2, MB4, MBg, MB]2 and Mg ) presented in Table 1, which shows correlations with the metallic radius r. values of which are in order of decreasing magnitude (r, corresponds to coordination number 12). In order to discuss the existence of the actinide borides, the table also shows the unit cell volume V of the borides MB4, MBg and MB,2. 

Inspection of Table 1 indicates that the unit cell volume of MB4, MB, MB,2 and MB borides varies as r except for the actinides. 

Table 3. Existence of the Actinides Borides Comparison of the Unit Cell Volume OF THE Actinide Tetraborides with Those of Comparable Rare-Earth Tetraborides...

Boron is as unusual in its structures as it is in its chemical behavior. Sixteen boron modifications have been described, but most of them have not been well characterized. Many samples assumed to have consisted only of boron were possibly boron-rich borides (many of which are known, e.g. YB66). An established structure is that of rhombohedral a-B12 (the subscript number designates the number of atoms per unit cell). The crystal structures of three further forms are known, tetragonal -B50, rhombohedral J3-B105 and rhombohedral j3-B 320, but probably boron-rich borides were studied. a-B50 should be formulated B48X2. It consists of B12 icosahedra that are linked by tetrahedrally coordinated X atoms. These atoms are presumably C or N atoms (B, C and N can hardly be distinguished by X-ray diffraction). 

Although BH3 may be the expected product, it is not stable as a discrete, monatomic unit. It is stable as an adduct with several Lewis bases. Some metals react with boron to form borides containing the... 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

The CuA12 tetragonal crystal structure, D j, 14/m cm is found for many AM2 type intermetallic and boride compounds (see Table 9.6). The tetragonal cell has four molecules per unit cell, a = 6.067 and... 

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