Nitride Formation and Atom Size

The second factor controlling the formation of nitrides is the atomic radius of the constituent elements. The radii of these elements are listed in Table 9.2 (see discussion on atomic radius in Sec. 4.1 of Ch. 2). One should note that nitrogen is one of the smallest atoms, and smaller than carbon. Table 9.2 also shows the type of nitride formed, i.e., interstitial (IS) or covalent (C), or intermediate (IM). 

Only the early transition metals (Groups IV, V, and VI) have a host lattice that is large enough for the nitrogen atom to fit readily and so form stable interstitial compounds, as shown in Table 9.1. As mentioned previously, only the nitrides of Group IV and V are considered refractory. 

The importance of the atomic radius will become evident as the structure of interstitial and covalent nitrides is reviewed in Chs. 10 and 12. Generally speaking, when the difference in radii of the two elements is large, interstitial nitrides are formed (i.e., TiN) when it is small, covalent nitrides are formed (i.e., Si3N4). 

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