Metal borides trigonal prisms

The structures of metal-rich borides can be systematized by the schematic arrangements shown in Fig. 6.6, which illustrates the increasing tendency of B atoms to catenate as their concentration in the boride phase increases the B atoms are often at the centres of trigonal prisms of metal atoms (Fig. 6.7) and the various stoichiometries are accommodated as follows ... 

Figure 6.6 Idealized patterns of boron catenation in metal-rich borides. Examples of the structures (a)-(f) are given in the text. Boron atoms are often surrounded by trigonal prisms of M atoms as shown in Fig. 6.7.

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 - " . 

With the exception of some special cases, two major classes of structure can be distinguished. In the first, the metal-rich borides possess boron atoms at the centers of trigonal prisms of metal atoms, and the boron atoms interact in one- or two-dimensional homonuclear networks. This class covers the range of stoichiometries from M3B to MB2. Presumably these structures, particularly those containing the highest metal-to-boron ratio, are controlled by the requirements of the metal... 

Fig. 9. (a) Arrangement of trigonal prisms in a close-packed structure showing a boron atom in one trigonal prismatic hole and the locations of the next nearest neighbor holes p, q, and r. (b) A three dimensional representation of the boron atom environment in (a), (c) A representation of the structure of the boride MB2, where all holes are filled and the boron atoms form a hexagonal net within the close-packed metal lattice. 

FIG. 24.7. The crystal structures of metallic borides (a) close-packed metal layer in UBj (AIB2) with B in trigonal prism holes between layers, (b) metal layer in UB4 with positions of 6- and 8-coordination between layers, (c) metal layer in CaB (ThBg) structure, (d) the UB4... 

In metal borides the metal atoms are not stacked as in the corresponding metal lattices (hcc, fee, or bcc) but form stacks to accommodate the boron nets or chains. In MB2 the metal atoms form face-linked trigonal prisms. The perovskite structure is rare in borides but it does occur in some ternary subborides, e.g., in LnRh3Bi in which Ln is a lanthanide. 

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