Metal-to-Carbide Structural Switching

As shown in Table 3.2, most early transition metals have a bcc structure which is not geometrically suitable to accommodate the carbon atoms in its interstices. Consequently, in order to form a carbide, the metal must switch to a close-packed structure (fee or hep), which provides octahedral sites large enough for the carbon atoms and is in turn stabilized by them. The switch can be seen by comparing Tables 3.2 and 3.4. 

Carbide M-M Spacing as Pure Metal (tim) M-M Spacing as Host Metal (nm) Change (%)  

This switch in the metal structure occurring when an interstitial carbide is formed has been used to assert that these compounds are not truly interstitial structures since the basic metal fiamework is modified.This interpretation may be semantically correct but the use of the term interstitial is widespread and, if not entirely accurate, provides a visual and easy-to-grasp representation of the structure. 

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