The Structures of Transition Metal Carbides

The structure of transition metal carbides are closely related to those of the transition metal nitrides. However, transition metal carbides feature generally simpler structure elements as compared to the nitrides. In carbides, the metal atoms are arranged in such a way that they form close-packed arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixture of these see Nitrides Transition Metal Solid-state Chemistr. The carbon atoms in these phases occupy the octahedral interstitial sites. A crystallochemical rule claims that the phases of pure h type can have a maximum carbon content of [C]/[T] = 1/2 and the c type phases a maximum carbon content of [C]/[T] = 1 hence in structures with layer sequences comprising h and c structure elements the maximum nonmetal content follows suit. 

THE CHEMISTRY OF TRANSITION METAL CARBIDES AND NITRIDES 7.2.1 Electronic structure of dicarbides... 

An extensive compilation of the crystal structures of transition metal carbides is found in Pearson s Handbook. Ward has discussed the structures of carbides extensively. Epicier and de Novion have suimnarized the results of investigations on ordering in transition metal carbides. Lengauer recently reviewed the knowledge on transition metal carbides and carbonitrides. For a comparison of close-packed transition metal carbides with close-packed transition nitrides, see Nitrides Transition Metal Solid-state Chem istry. 

The effects on the properties of transition metal carbides and nitrides produced by metal sublattice doping have been studied for quite an extended period. So far, large groups of ternary (and more complex) carbide and nitride alloys of the M M i (C,N) type, where M, M are metals, have been synthesised (see Goldschmidt (1967) or Samsonov, Upadkhaya and Neshpor (1974)). In this chapter we review the results of researches on the electronic structure and interatomic bonding in such solid solutions. 

B. Theoretical Methods Applied for the Investigation of the Influence of Vacancies on the Electronic Structure of Transition Metal Carbides and Nitrides with B1 Structure... 

This volume, which is unique in its coverage, provides a general introduction to the properties and nature of transition metal carbides and nitrides, and covers their latest applications in a wide variety of fields. It is directed at both experts and nonexperts in the fields of materials science, solid-state chemistry, physics, ceramics engineering and catalysis. The first chapter provides an overview, with other chapters covering theory of bonding, structure and composition, catalytic properties, physical properties, new methods of preparation, and spectroscopy and microscopy. 

Vines F, Sousa C, Liu P, Rodriguez JA, lllas F (2005) A systematic density functiona theory study of the electronic structure of bulk and (001) surface of transition-metals carbides. JChem Phys 122 174709... 

These observations on the structurally simple carbides of the early transition metals show how the strength of binding of core carbon atoms in molecular metal carbonyl clusters can in principle be estimated by comparison with metal carbides for which structural and theoretical data are available, and leads us to hope that examination of the wider body of transition metal carbides will provide relationships between the length and strength of bonds between metal atoms and octahe-drally coordinated carbon atoms that can be applied to specific molecular metal carbonyl clusters containing core carbon atoms. 

Yvo] Yvon, K., Parthe, E., On the Crystal Chemistry of the Close Packed Transition Metal Carbides, 1. The Crystal Structure of the -V, Nb, and Ta Carbides , Acta CrystalL, B26(l), 149-153 (1970) (Crys. Structure, Experimental,, 16)... 

Up to now, we have discussed mostly model intermetallic compounds with simple crystal structures (generally cubic LI2, B2,. . . ) and containing two metal species. We shall now present briefly some properties of point defects in more exotic systems, of considerable interest the A15 superconductors, transition-metal carbides and nitrides, and III-V semiconductors (e.g. GaAs). 

Table 1 Features of Transition Metal Carbides with the NaCl-Type Structure ...

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

The metal not discussed yet is iron. It appeared to be a rather inactive metal. The possible reason for this is that iron is, under a running skeletal reaction or under conditions when more difficult dehydrogenation/hydro-genation can occur, covered by carbon to such an extent that one can rather speak of Fe carbides being the catalyst here. Most likely, the same holds for Group III—VI transition metals. However, carbides (with an imperfect structure) of these metals are, in contrast to Fe, active in skeletal reactions. 

Both the controversy initiated through these early investigations, and the fundamentally interesting properties of the transition metal carbides and nitrides, have stimulated tremendous interest in providing a model for the bonding in these materials. As electronic structure calculations have become more common as a tool in the study of solid state properties, numerous models have been proposed.12 19... 

The transition metal carbides (TMC) are interesting because of their prominent properties such as great hardness, high electrical and thermal conductivities, stable field-electron emission1 and efficient catalysis.2 These properties are closely related to their electronic structures, yet the Fermi surfaces of TMC are not yet well established experimentally. In the case of hexagonal tungsten carbide (WC), there is only one reported experiment on the observation of de Haas-van Alphen oscillations.3... 

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