Group IV nitrides

As shown in Fig. 11.2, tfaennal expansion is essentially linear with increasing temperature (data available for Group IV nitrides only).l l... 

As shown in Table 11.5, the hardness of the interstitial nitrides is somewhat lower than that of the corresponding carbides. The Group IV nitrides generally have higher hardnesses than those of Groups V. This reflects the greater contribution of M-N bonding found in these compounds. 

Isomorphism. HfN is completely and mutually soluble with the nitrides and carbides of Groups IV and V with the exception of VN and VC (see Ch. 7). 

Carbides oxidize readily although less rapidly than the nitrides but more so than the borides. Oxidation becomes more rapid going from the Group IV carbides (TiC, ZrC, HfC) to those of Group VI (Cr3C7, MoC, WC). In some cases, a protective film of the metal oxide is formed. Such is the case with SiC, as reviewed in Sec. 5.7 below. 

Tungsten carbide — WC, belongs to a class of Group IV B-VIB transition metal carbides and nitrides, often referred to as interstitial alloys, in which the carbon and nitrogen atoms occupy the interstitial lattice positions of the metal [i]. These compounds possess properties known from group VIII B precious metals like platinum and palladium [ii]. Thus, they show remarkable catalytic activities, attributed to a distinct electronic structure induced by the presence of carbon or nitrogen in the metal lattice. Tungsten carbide resembles platinum in its electrocatalytic oxidation activity (- electrocatalysis) and is therefore often considered as an inexpensive anode electrocatalyst for fuel cell [iii] and -> biofuel cell [iv] application. 

The transition metals of groups IV, V and VI, particularly Ti, V, Cr, Mo, and W, have the strongest N2-reducing capacity. Ti compounds are particularly active. In the first row of transition metals, the ammonia yields decrease generally from left to right, in line with the decreasing stability of the metal nitrides. Co and Ni compounds are usually of low or no activity. Palladium, copper and platinum complexes have no activity in any system tested. 

In discussing bonds formed between the group 14 elements and nitrogen, two compounds of particular importance emerge cyanogen, C2N2, and silicon nitride. Tin(IV) nitride has recently been prepared. 

IV. POLYMERIC ROUTES TO MAIN GROUP ELEMENT NITRIDES... 

In antiperovskite nitrides of the type Mn N, Fe N and Fc3PtN, nitrogen is present in the octahedral holes of the metal framework. Chem et al. [10] prepared a new antiperovskite nitride of the formula Ca3MN where M is a group IV or V element. Here the A and B sites are occupied by M and N, respectively and the anion sites by Ca ". These air-sensitive nitrides are prepared by grinding CajN (obtained by the... 

In this section and the next three, the properties and characteristics of the interstitial carbides of Group IV are reviewed and compared with those of the host metals, the corresponding interstitial nitrides, as well as those of another refractory group the borides of the Group IV metals. The values given are those for composition as close to stoichiometry as possible. 

As shown in the above table, the Group IV carbides (and Groups V and VI carbides as well) are good electrical conductors and have an electrical resistivity only slightly higher than that of the parent metals, reflecting the metallic character of these compounds. The nitrides and especially the borides have even lower resistivity. The large spread in the reported values may be attributed to differences in composition and the presence of defects and impurities. 

Many ternary carbides and nitrides are known and some of these compounds have excellent properties. For instance, the hardness of ternary-carbide systems of the same group (Group IV or Group V) is considerably higher than the hardness of the binary constituents.1 1 A hardness of approximately 43.1 GPa is reported for the compound TIq 4C... 

Figure 4.8 Mutual solubilities of Group IV and Group V carbides and nitrides.

Hardness. Table 5.6 shows that carbides are die hardest, followed by the borides and the nitrides. The Group V carbides have higher hardness than those of Group VI but are not quite as hard as those of Group IV (see Ch. 4, Sec. 4.4 and Ch. 6, Sec. 4.0). This reflects the intermediate strength of M-C bonds found in these carbides. 

The existence of ternary carbides and nitrides was discussed in Ch. 4, Sec. 5.0. As shown in Fig. 4.7 (Ch. 4), VC, NbC, and TaC have complete mutual solubility and variable solubility with the carbides of Group IV. With the partial exception of VC, they are also mutually soluble with the nitrides of Groups IV and V (see Fig. 4.8).O i... 

In the five categories listed above, only some of the interstitial and covalent nitrides qualify as refractory, i.e., the nitrides of the elements of Groups IV and V and the covalent nitrides of boron, aluminum, and silicon. These elements are shown in bold type in Table 9.1. Unlike the carbides of Group VI elements, the Group VI nitrides are not refractory and consequently are not considered in any depth in this book. 

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. 

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