Cubic carbides of Ti, V, Zr, Nb, Hf and Ta

Stoichiometric cubic (NaCl-type) carbides of iva and va transition metals (referred to as d-metal carbides) have received most attention in the literature. Due to the simplicity of their crystal structure, these phases (especially TiC and VC) have been studied by a large number of quantum-chemical methods, as well as by X-ray emission, electron. 

First we consider the main features of the electron energy spectrum of these compounds taking TiC as an example. Fig. 2.1 shows the dispersion curves for TiC as obtained from the self-consistent APW calculations of Neckel et al (1976). Characteristic elements of the valence-state spectrum of the carbide are the following  

Such a distribution of valence states in TiC follows from the results of all band and cluster calculations. Depending on the calculation technique and the approximations used the results obtained in some particular papers may differ a little in the widths of some bands, the energy intervals between the bands, the position of Fermi level and the details of the fine structure of the density of states (DOS). 

The main features of chemical bonding follow directly from the 

As soon as the metal atoms in the carbides have octahedral coordination, one can break metal d states into t2g(d y,d, dy ) and Cgid and dl — y ) orbits. The calculations show that the Cg states contribute most to the hybridisation band and the t2g states to the metal band. This makes it evident that the main part of the chemical bonding in such compounds 

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