Metallic carbides electronic structure

The influence of the B impurity on the carbide electronic structure (Fig. 5.1) leads to the appearance of an additional B2s-Iike peak near the lower edge of the p-d-band in the TiC-B spectrum. The DOS near the Fermi level also increases, while the energy width of occupied metal states is somewhat enhanced at the expense of Ti orbitals mixed with the B states. 

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 " . 

Semiconducting Properties. Silicon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stability of its electronic structure, silicon carbide has been studied for uses at high (>500° C) temperature. The Hall mobility in silicon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-type crystals, activation energy for ionization of nitrogen impurity varies with polytype (50,51). 

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

Figure 1.3 Electronic structure of transition metal carbides and nitrides. Band calculation reproduced from Hasegawa (Ref. 18).

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... 

In considering the effect of the electronic structure of catalysts on activity, Dowden (33) suggested that carbides, and similarly nitrides and carbonitrides, should be less active for synthesis than the corresponding metal since the interstitial atoms may contribute electrons to the unfilled d-shells of the metal, which are believed to be essential for the catalytic activity of transition metals in hydrogenation reactions. This hypothesis is supported by the low activity of cobalt carbide compared with that of reduced cobalt (28,29). For iron catalysts the hypothesis... 

Tungsten carbide — WC, belongs to a class of Group IV B-VIB transition metal carbides and nitrides, often referred to as interstitial alloys, in which the carbon and nitrogen atoms occupy the interstitial lattice positions of the metal [i]. These compounds possess properties known from group VIII B precious metals like platinum and palladium [ii]. Thus, they show remarkable catalytic activities, attributed to a distinct electronic structure induced by the presence of carbon or nitrogen in the metal lattice. Tungsten carbide resembles platinum in its electrocatalytic oxidation activity (- electrocatalysis) and is therefore often considered as an inexpensive anode electrocatalyst for fuel cell [iii] and -> biofuel cell [iv] application. 

Electronic structure calculations for transition metal carbides (Neckel 1990, Le 1990, Le et al. 1991) reveal significant contributions to cohesion by all three main types of chemical bonding. Covalent bonds are due to the formation of molecular orbitals by combining atomic d-orbitals of the metal with p-orbitals of C. Ionic bonds result from charge transfer from the metal to the non-metal. Metallic bonds are due to s electrons and also to a non-vanishing density of d-p electronic states (DOS) existing at the Fermi level (Figure 7.30). The main difference between the DOS curves calculated for stoichiometric ZrC, TiC or HfC and NbC, TaC or VC is... 

Borides Sohd-state Chemistry Carbides Transition Metal Solid-state Chemistry Electronic Structure of Sohds Quasicrystals Structure Property Maps for Inorganic Solids Superconductivity Zintl Compounds. 

Alloys Borates Solid-state Chemistry Carbides Transition Metal Solid-state Chemistry Chalcogenides Solid-state Chemistry Diffraction Methods in Inorganic Chemistry Electronic Structure of Solids Fluorides Solid-state Chemistry Halides Solid-state Chemistry Intercalation Chemistry Ionic Conductors Magnetic Oxides Magnetism of Extended Arrays in Inorganic Solids Nitrides Transition Metal Solid-state Chemistry Noncrystalline Solids Oxide Catalysts in Solid-state Chemistry Oxides Solid-state Chemistry Quasicrystals Semiconductor Interfaces Solids Characterization by Powder Diffraction Solids Computer Modeling Superconductivity Surfaces. 

The types of molecules considered in this work are those that have structural or chemical features that are manifestly different than are those of their more common oxidation state counterparts. Because of the breadth of this subject, selected examples are presented to illustrate typical behavior. The properties of the types of compounds containing the elements in more typical oxidation states may be found in the Inorganic and Organometallic sections describing each element or gronp and will not be discussed in this article. Similarly, minerals, metal phosphides, metal carbides, and compounds where the oxidation state of the element is low based on formal electron counting techniques (as in some catenated Catenation group 14 compounds), but that do not result in unusual chemistry, are not included. 

Electronic Structures of Metal Carbides TiC and UC Similarity and Dissimilarity... 

To elucidate the nature of chemical bonding in metal carbides with the NaCl structure, the valence electronic states for TiC and UC have been calculated using the discrete-variational (DV) Xa method. Since relativistic effects on chemical bonding of compounds containing uranium atom become significant, the relativistic Hamiltonian, i.e., the DV-Dirac-Slater method, was used for UC. The results... 

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