The Crystal Structure of Borides

Similar to silicates, the crystal structures of borides can easily be classified according to the arrangement of the boron atoms. Boron may occur as an isolated atom or form B-B bonds with an increasing degree of interconnection in the chains, double chains, layers and frameworks and combinations thereof (Fig. 1). Due to the strong covalent bonding between the boron atoms and the electron deficiency of the three-center bond a number of complex and unique structures result which 

Some general systematics on chemical bonding and crystal chemistry have been published by Matkovich and Economy [7], and Aronsson et al. [8, 9], who also refer to the structural similarities in silicides and phosphides. 

Calculation of the band structures of AlB2-type compounds shows that no band gaps are present, and all the compounds are predicted to be electron conductors, which is in agreement with experimental results. For the main-group element 

As it has been established that the boron network is rather rigid and governs the lattice expansion in the a direction whereas the lattice dilatation perpendicular to the metallic layers strongly depends on the metal species, it seems likely that the metal atoms are distorted in some cases [21, 22]. The cja ratio is thus a function of the metai/ boron raHo and depends furthermore on the valency electron concentration [19]. 

2 Crystal Structure of Boron Carbide and Isotopic Compounds 

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