Transition Element Nitrides

The instability ol the later transition element nitrides such as Co3N2 and Ni3N2 (compared to say the readily formed Mg, N2) is more readily understood in terms of the greater cohesive energy of the metallic elements. (The heat of sublimation of Mg at OK is 145 kJmol I, while for Ni it is 428 kJmol 1). 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

We note that the valence orbitals of metal atoms order in energy as AE>Ln>M. The d-levels of transition elements (M) range the lowest, and are therefore most sensitive for reduction, or to form a stable binary metal nitride. This may also explain the virtual absence of d-element compounds with 16 (valence) electron species, such as [N=N=N] , [N=C=N] , [N=B=N] T [C=C=CfT or [C=B=C] T at least through high-temperature syntheses. 

Hagg, G. 1931. Regularity in crystal structure in hydrides, borides, carbides and nitrides of transition elements. Z. Physik. Chem. 12B 33-56. 

Table 2. Heat and free energy of formation from the elements of transition metal nitrides (kJ mol-1) calculated from data in Ref. 22.

The stmeture of transition metal carbides are closely related to those of the transition metal nitrides. However, transition metal carbides feature generally simpler stmeture elements as compared to the nitrides. In carbides, the metal atoms are arranged in such a way that they form close-packed arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixtme of these (see Nitrides Transition Metal Solid-state Chemistry). The carbon atoms in these phases occupy the octahedral interstitial sites. A crystallochemical rule claims that the phases of pure h type can have a maximum carbon content of [C]/[T] = 1/2 and the c type phases a maximum carbon content of [C]/[T] = 1 hence in stractures with layer sequences comprising h and c stractme elements the maximum nonmetal content follows suit. 

The hydrid( S, borides, carbides and nitrides of the transitional elements have metallic properties. Only atoms with small covalent radii are capable of occupying the interstices in relatively close-packed arrangements ... 

We suspect that the lack of alkali nitrides on the one hand and of transitional metal nitrides on the other hand led to the neglect, until recently, of a very rich ternary nitride chemistry. We have recorded here some remarkable nitrides (mostly reported rather recently) containing transition metal and alkali metals. We suspect that some of them were made serendipitously, as crucibles of elements like iron are often used to contain alkali-rich materials (such as Li, N). We observe that many ternary etc. oxides of elements such as Pt were similarly discovered as products of reactions in precious metal crucibles. 

V. POLYMERIC ROUTES TO TRANSITION ELEMENT CARBIDES AND NITRIDES... 

Transition element carbides and nitrides are applied as cutting tools because of their extreme hardness and wear resistance. In some cases nitrides and carbides (e.g., of titanium) form solid solutions over the entire compositional range other transition metal nitrides and carbides exhibit fairly different structures and are not completely soluble. Carbon contents within the range of few percentage points usually do not influence the mechanical properties of transition metal nitrides, and vice versa. Hence, completely carbon-free nitrides or nitrogen-free carbides are not required, especially for the titanium compounds. 

Until now, polymeric routes to transition metal nitrides or carbides have not been as numerously reported in the literature as those of the main group elements. They have been developed in most cases to produce thin coatings on various substrates or fibers. 

All precursors are amorphous up to calcination temperatures of around 600°C. At higher temperatures, in most cases powders with extremely small crystallite sizes of around 20-40 nm are formed (Fig. 7). A further increase in calcination temperature promotes crystal growth. With aluminum nitride, a white powder with a low oxygen and carbon content is obtained [97]. Other main group element precursors exhibit fairly different behaviors Mg and Ca precursors yield metal cyanamide [99]. Calcination of the transition element precursors (Fig. 8) results in the formation of nitrides, carbonitrides, or carbides. For the titanium-containing precursors, TiN/TiC solid solutions can be obtained [96] the quantity of carbon strongly depends on the calcination atmosphere applied (argon, 31 wt% ammonia, 5.1 wt%). 

As shown in subsequent chapters, most elements form carbides and nitrides and these can be divided into several types with different physico-chemical structures and characteristics. Of these, however, only the interstitial and covalent materials meet the refractory qualification. This includes the carbides and nitrides of the nine transition elements of Groups rv, V, and VI and the 4th, 5th, and 6th Periods, the carbides and nitrides of boron and silicon, and aluminum nitride. 

The metals of the nine early-transition elements, i.e., titanium, zirconium, and hafiiium of Group IV, vanadium, niobium, and tantalum of Group V, and chromium, molybdenum, and tungsten of Group VI, fit the criteria of size and site availability, and form interstitial nitrides.1 1... 

It is well known that the majority of refractory carbides and nitrides exhibit superconductivity. For NaCl-type compounds the critical temperature of transition to the superconducting state rises with the increase of transition element group number. This spurred a number of researchers to study the band structure of a series of nitrides (VN, NbN, TaN, CrN, MoN, WN,...) and to calculate the temperature with the objective of finding the compounds with the maximum - see Chapters 2 and 3. 

Well known sintering aids in category (1) are alkali-earth oxides or rare-earth oxides such as Y2O3 (17,18) and CaO (19,20). These can be added not only as oxide but also as nonoxide compounds such as halide, nitride, carbide, nitrite, or carbonate. Some of the transition elements such as NiO and Ti02 can be classified as category (2) additives. Rare earth or alkali-earth oxide additive reacts with aluminum oxide of AIN powder (i.e. oxide layer of AIN powder) to form aluminate liquid at a high temperature Eq. (4) and promotes liquid-phase sintering of AIN powder. 

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