How C-C Bonds Are Formed and How They Influence Structural Choices in Some Binary and Ternary Metal Carbides

The dimeric C₂ unit has been found in many binary and ternary metal carbide systems. The C-C bond length in these crystal compounds varies over a wide range, from a conventional carbon-carbon single bond to a typical C-C triple bond. The formation of C₂ units is an important factor affecting the structural stability as well as other physical properties. In the UC₂ structure, both uranium-carbon bonding and carbon-carbon bonding are enhanced upon formation of such dimeric units and the system is greatly stabilized. Our calculations indicate that UC₂ is metallic, whereas the alkaline-earth metal carbide CaC₂, a structure belonging to the same crystal family, has a substantial gap between the valence and conduction bands. The pairing of carbon atoms in DyCoC₂ structure derives from a Peierls-type distortion. This crystal form should be favorable for electron counts of 19 → 21, with late transition-metal elements. Early transition-metal carbides of the same composition do not exist in this form and a rationale is given for this. UCoC₂, another stable form of RTC₂, also contains short C-C bonds. Structurally and electronically, comparisons are made of this structure and a closely related UCuAs₂ type, in which carbon atoms do not form bonds. It is found that the UCoC₂ structure is favorable for a valence electron count of 23 or less, whereas the UCuAs₂ form becomes more stable for 24 electrons or more. Finally, we discuss briefly two possible carbide forms of the ThCr₂Si₂ type structure.