How C-C bonds are formed and how they influence structural choices in some binary and ternary metal carbides

Jing Li, Roald Hoffmann

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Abstract

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.

Original languageEnglish
Pages (from-to)83-101
Number of pages19
JournalChemistry of Materials
Volume1
Issue number1
Publication statusPublished - 1989

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ASJC Scopus subject areas

  • Materials Chemistry
  • Materials Science(all)

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