Interstitial carbides structure

As mentioned in Ch. 9, the refractory nitrides consist of two structurally different types generally known as interstitial and covalent nitrides. This chapter provides a general review of the structural characteristics and composition of the interstitial nitrides and follows the outline of Ch. 3, Interstitial Carbides Structure and Composition. Some of these interstitial nitrides, titanium nitride in particular, are major industrial materials. 

The interstitial carbides are compounds formed by the direct reaction of a d-block metal and carbon at temperatures above 2000°C. In these compounds, the C atoms occupy the gaps between the metal atoms, as do the H atoms in metallic hydrides (see Fig. 14.9). Here, however, the C atoms pin the metal atoms together into a rigid structure, resulting in very hard substances with melting points often well above 3000°C. Tungsten carbide, WC, is used for the cutting surfaces of drills, and iron carbide, FesC, is an important component of steel. 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

Tantalum carbide (TaC) is arefractory interstitial carbide with a high melting point. It is structurally and chemically similar to niobium carbide. It has two phases Ta and the monocarbide TaC. Thelatteris the only phase of industrial importance and the only one described here. The characteristics and properties of TaC are summarized in Table 9.7. 

The crystal structure and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, rx / rM < 0.59. When this ratio is larger than 0.59, as in the Group 7—10 metals, the structure becomes more complex to compensate for the loss of metal—metal interactions. Although there are minor exceptions, the H gg rule provides a useful basis for predicting structure. 

FIGURE 14.48 The structure of an interstitial carbide, in which the carbon atoms (represented by the black spheres) lie between metal atoms (the gray spheres), thereby producing a rigid structure. 

Compounds with the sodium chloride structure range from the essentially ionic halides and hydrides of the alkali metals and the monoxides and monosulphides of Mg and the alkaline-earths, through ionic-covalent compounds such as transition-metal monoxides to the semi-metallic compounds of B subgroup metals such as PbTe, InSb, and SnAs, and the interstitial carbides and nitrides (Table 6.1). Unique and different distorted forms of the structure are adopted by the Group IIIB... 

The main structural features of the transition-metal interstitial carbides are as follows. The maximum carbon content depends on the c.p. layer sequence. The two octahedral interstices on either side of an h layer are located directly above one another, and only one of these is ever occupied. This restriction gives the following limiting formulae ... 

Metal-like and Salt-like Interstitial Compounds. A different type of interstitial lattice structure—that of calcium carbide— is shown in Fig. 87, in which the lace-centred cubic calcium atoms are shown as ), and the interstitial carbon atoms are in groups of two. This type of lattice, in view of its slight vertical distortion, is face-centred tetragonal rather than face-centred cubic, but its most interesting feature is tho fact that here the interstitial carbon a,toms are linked together in groups of two. 

Another view of the FeMoco compares it to bulk metals with interstitial carbide, nitride, or oxide in the lattice. It is difficult to draw clear analogies between the band electronic structure of solids (through which oxide, nitride, and carbide materials are understood) and the localized interactions in the FeMoco, although theoretical calculations should give progress along these lines. 

Whereas certain bonding features are considered most unusual in a molecule, similar features are quite normal in solid state compounds, lb give an example, the unusual coordination of the carbon atom in the discrete molecule [ligQ is suggestive of hypervalency, [1, 2] however the occurrence of octahedrally coordinated carbon atoms is often observed in transition metal carbides having the rocksalt type structure, thus a quite normal situation. From a simple ionic representation as [(Ii )5C (e )2], the only surprise is the surplus of electrons. Yet, even this feature is familiar with such interstitial carbides as V2C in which, the valence electrons of V (li) which are not used for heteronuclear V-C (li-Q bonds are used to form homonuclear V-V (li-Ii) bonds, as suggested by the (8-N) rule. [3]... 

This chapter is a general review of the structural characteristics of the refractory carbides, their classification, and general features. These materials can be divided into two major types the interstitial carbides reviewed in Chs. 3 to 6, and the covalent carbides, reviewed in Chs. 7 and 8. 

The importance of the atomic radius will become evident as the structure of interstitial, intermediate, and covalent carbides is reviewed in Chs. 3 (intersitial carbides) and 7 (covalent carbides). Generaly speaking, when the difference is large, interstitial carbides are formed (i.e., TiC) when it is small, covalent carbides are formed (i.e., SiC). 

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