Hexagonal close-packed carbide

The identity of the Hagg carbide was established by the x-ray diffraction pattern which agreed with that reported (Hagg, 50). The identity of the hexagonal close-packed carbide was established by indexing the diffraction pattern. 

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. 

Besides the trigonal prismatic coordination of the carbon atoms, octahedral coordination is observed, especially for the carbon atoms in the carbides of the early transition metals [46]. Important examples are the NaCl type carbides TiC, ZrC, HfC, VC, NbC, and TaC [42], where the metal atoms have a cubic close-packed arrangement, and the tungsten carbide W2C, where the tungsten atoms form a hexagonal close packed structure [47,48]. 

The layer sequences can repeat themselves in the cycles ABC, ABC. .. (zinc blende, type 3C) or AB, AB. .. (wurtzite, type 2H), according to cubic or hexagonal close packing. In addition, numerous others stack sequences are formed in the case of silicon carbide, resulting in many similar polytypes. 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

Silicon carbide exhibits a two-dimensional polymorphism called polytypism. All polytypes have a hexagonal frame of SiC bilayers. The hexagonal frame should be viewed as sheets of spheres of the same radius and the radii touching, as illustrated in Figure 1.5. The sheets are the same for all lattice planes. However, the relative position of the plane directly above or below are shifted somewhat to fit in the valleys of the adjacent sheet in a close-packed arrangement. Hence, there are two inequivalent positions for the adjacent sheets. 

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. 

Figure 12. Stacking sequences of the hexagonal metal atom layers in several carbides with close packed transition metals [47]. The structures are represented by cuts along the (110) planes of the respective hexagonal cells. The transition metal (7) and carbon atoms are drawn as large open and small filled circles, respectively. A forbidden carbon position, discussed in the text, is marked with an asterisk in the structure of T2C.

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. 

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