Metal boride classification

Phosphides resemble in many ways the metal borides (p. 145), carbides (p. 297), and nitrides (p. 417), and there are the same difficulties in classification and description of bonding. Perhaps the least-contentious procedure is to classify according to stoichiometry, i.e. (a) metal-rich phosphides (M/P > 1), (b) monophosphides (M/P =1), and (c) phosphorus-rich phosphides (M/P < 1) ... 

Table 12.3 Classification of the structures of solid state metal borides. 

Figure 1 Classification of metal borides with respect to the boron aggregation as a function of the boron to metal ratio in ternary metal borides - the prevailing metal coordination of the B - atom is outlined.

This kind of classification has been used by Nowotny and Rogl (1977) and Rogl and Nowotny (1978) in a survey of ternary metal borides. The structures have been classified according to the occurrence of isolated B atoms, B-B dumbbells, B chains, or B nets and so on. A similar classification of binary transition metal borides had been proposed many years before by Kiessling (1950). The type of homonuclear boron linkage can be correlated with the metal to boron ratio of the compound (see table 20). 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

Metal boron ratio Code Boride Pearson s classif. symbol Space group Type of boron linkage Fig. 

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