Tungsten carbide crystals structure

Figure 3.16 Schematic ie M esentation of the simple hexagonal (hex) structure of the tungsten carbide crystal.

Tungsten carbide has a complex crystal structure with three phases Wq (subcarbide), the monocarbide WC (also called a-WC), and P-WCj.x, which is unstable and forms only above 1530°C. The monocarbide WC is the most important phase and the one reported here. Its characteristics and properties are summarized in Table 9.9. 

The crystal structures adopted by the binary carbides and nitrides are similar to those found in noble metals. The resemblance is not coincidental, and has been explained using Engel-Brewer valence bond theory [5]. Briefly, the main group elements C and N increase the metal s effective s-p electron count, so that structures and chemical properties of the early transition metals resemble those of the Group 8 metals. This idea was first introduced by Levy and Boudart [6] who noted that tungsten carbide had platinum-like properties. 

Boj] Bojarski, Z., Leciejewicz, J., Neutron-Diffraction Study of the Crystal Structures of eta-Type Tungsten Iron Carbides (in Polish), Arch. Hutn., 12(3), 255-263 (1967) (Crys. Stracture, Experimental, 9)... 

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