Close-packed interstitial carbide

Closed-Packed Structures. In a close-packed interstitial carbide, the carbon atom is far too large to occupy a tetrahedral site and can only fit into an octahedral site. In these sites, it is octahedrally coordinated with the six metal atoms that surroimd it and thus achieves the highest possible coordination number. Since there is only one octahedral site per metal atom and if all are occupied by a carbon atom, a stoichiometric monocarbide is formed. 

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. 

We have shown that A) interstitial hydride formation is observed only with partial occupation of the available holes, B) occupation of the interstitial position in isolated polyhedra is not observed, and C) occupation of all the holes in a close-packed lattice cancels metal-metal interactions. Therefore, it seems that interstitial hydrogen can be tolerated only in a fraction of the total number of holes, and with the weakening of metal-metal interactions. This behavior indicates strong competition between metal-metal and metal-hydrogen bonds, which is unique for hydrogen because interstitial carbon can stabilize some unusual arrangements in carbonyl carbide clusters (29, 30). 

The monometallic carbides and nitrides often adopt simple crystal structures (Fig. 1 ) with the metal atoms arranged in cubic close-packed (ccp), hexagonal close-packed (hep) or simple hexagonal (hex) arrays. The nonmetallic elements, C, N, and O, occupy interstitial spaces between metal atoms, and for this reason the materials are also known as interstitial alloys. 

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. 

Although it is more stable than Ti3N4, strong heating converts it to ZrN. The interstitial nitrides, like the carbides TiC and ZrC, have the NaCl structure alternatively they may be regarded as cubic close-packed arrangements of Ti atoms with nitrogens in the octahedral holes. As the metals have h.c.p. lattices, these have not been simply expanded to admit the N atoms. 

A. .. A etc. contacts between the layers (see later). The reason for the great importance of the most closely packed structures is that in many halides, oxides, and sulphides the anions are appreciably larger than the metal atoms (ions) and are arranged in one of the types of closest packing. The smaller metal ions occupy the interstices between the c.p. anions. In another large group of compounds, the interstitial borides, carbides, and nitrides, the non-metal atoms occupy Interstices between c.p. metal atoms. 

In the so-called interstitial nitrides the metal atoms are approximately, or in some cases exactly, close-packed (as in ScN, YN, TiN, ZrN, VN, and the rare-earth nitrides with the NaCl structure), but the arrangement of metal atoms in these compounds is generally nor the same as in the pure metal (see Table 29.13, p. 1054), Since these interstitial nitrides have much in common with carbides, and to a smaller extent with borides, both as regards physical properties and structure, it is convenient to deal with all these compounds in Chapter 29. 

In table 29.13 are listed the structures of compounds MC and MN with metallic character and known crystal structure. Of all the metals in Table 29.13 forming a carbide MC or a nitride MN with the NaCl structure only four have the cubic close-packed structure. For all the other compounds the arrangement of metal atoms in the compound MX is different from that in the metal itself. (This is also true of many hydrides—see p. 294.) Also, although many of these compounds exhibit variable composition, some do not, for example, UC, UN, and UO. In any case, variable composition is not confined to interstitial compounds—see, for example, the note on non-stoichiometric compounds, p. 5. 

Interstitial carbides are formed by direct reaction of a o -block metal with C at temperatures above 2000°C. The C atoms lie in holes in close-packed arrays of metal atoms. Bonds between the C and metal atoms stabilize the lattice. Examples WC, Cr3C, and Fe3C, a component of steel... 

A curious compromise is reached in many ionic crystals. The crystal NaCl,f or example, is based on two interpenetrating close-packed (fee) lattices. The positions of one lattice are occupied by positive ions, while those of the other are occupied by negative ions. Consider the unit cube of the fee structure in Fig. 27.7(a). There is a void, or hole, outlined by the octahedron, at the center of the cube. An identical octahedral hole is centered on each edge of the unit cube (Fig. 27.7b). Each hole is at the center of an octahedron, which has atoms at each of the six apices. The centers of the octahedral holes occupy the positions of an fee lattice, which interpenetrates the lattice on which the atoms are located. Small foreign atoms, such as H, B, C, N, can occupy these holes. Many carbides, hydrides, borides, and nitrides of the metals are interstitial compounds formed in this way. 

In the transition metal carbides and nitrides, think of the metal as being in the close-packed arrangement with the carbon or nitrogen atoms located in interstices. The coordination number can again be determined by the radius ratio, which in this case is given by rjr where is the radius of the interstitial atom and is the radius of the metal atom. 

The transition metal carbides and nitrides have often been called interstitial compounds [70] however, this is somewhat misleading. The small boron, carbon, or nitrogen atoms certainly occupy octahedral or trigonal prismatic voids of the metal sublattice, but the arrangement of the metal atoms themselves is different from that of the element. In the monocarbides the transition metal atoms show cubic close packing. However, titanium, zirconium, and hafnium are packed hexagonally and vanadium, niobium, and tantalum are body centered cubic [1]. Thus, these monocarbides are inorganic compounds with their individual crystal structures and they should not be considered as an interstitial compound of a transition metal host lattice. 

Figure 1. Hexagonal (h) and cubic (c) stacking sequence of metal atoms of close-packed h.c.p. and f.c.c. transition metal carbides and nitrides. Open large circles designate metal atoms, filled small circles designate interstitial nonmetal. In h sequences only 50% of the interstices can be occupied. Nitrogen and carbon atoms are randomly distributed among the nonmetal sublattice.

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