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

Journal Article · Wed Feb 01 00:00:00 EST 1989 · Chemistry of Materials; (USA)

DOI:https://doi.org/10.1021/cm00001a019· OSTI ID:5247751

Li, Jing; Hoffmann, R ^[1]

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1. Cornell Univ., Ithaca, NY (USA)

The dimeric C{sub 2} 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 C{sub 2} units is an important factor affecting the structural stability as well as other physical properties. In the UC{sub 2} 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 UC{sub 2} is metallic, whereas the alkaline-earth metal carbide CaC{sub 2}, a structure belonging to the same crystal family, has a substantial gap between the valence and conduction bands. The pairing of carbon atoms in DyCoC{sub 2} structure derives from a Peierls-type distortion. This crystal form should be favorable for electron counts of 19 {yields} 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. UCoC{sub 2}, another stable form of RTC{sub 2}, also contains short C-C bonds. Structurally and electronically, comparisons are made of this structure and a closely related UCuAs{sub 2} type, in which carbon atoms do not form bonds. Is found that the UCoC{sub 2} structure is favorable for a valence electron count of 23 or less, whereas the UCuAs{sub 2} form becomes more stable for 24 electrons or more. Finally, we discuss briefly two possible carbide forms of the ThCr{sub 2}Si{sub 2} type structure.

• OSTI ID: 5247751
• Journal Information: Chemistry of Materials; (USA), Vol. 1:1; ISSN 0897-4756
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
COBALT CARBIDES
CRYSTAL STRUCTURE
ELECTRONIC STRUCTURE
DYSPROSIUM COMPOUNDS
URANIUM CARBIDES
BAND THEORY
BINDING ENERGY
BOND ANGLE
BOND LENGTHS
CARBON
CHEMICAL BONDS
DIMERS
ENERGY GAP
EXPERIMENTAL DATA
LATTICE PARAMETERS
VALENCE
X-RAY DIFFRACTION
ACTINIDE COMPOUNDS
CARBIDES
CARBON COMPOUNDS
COBALT COMPOUNDS
COHERENT SCATTERING
DATA
DIFFRACTION
DIMENSIONS
ELEMENTS
ENERGY
INFORMATION
LENGTH
NONMETALS
NUMERICAL DATA
RARE EARTH COMPOUNDS
SCATTERING
TRANSITION ELEMENT COMPOUNDS
URANIUM COMPOUNDS
360202* - Ceramics
Cermets
& Refractories- Structure & Phase Studies