Chemical Bonding in Transition Metal Carbides PDF

PREFACE

Transition Metal carbides are scientifically interesting, as well as being industrially important. The affinity of early transition metals such as titanium for carbon leads to compounds with simple crystal structure, high melting point and great hardness. The simplicity of structure is not maintained in the carbides of the later metals.

The superficial clues to the nature of the chemical bonding in these carbides are quite misleading. The NaCI-type structure, which is endemic among the carbides of the early metals, suggests an ionic compound; but they are metallic and the carbon is covalently bonded to the metal. The metallic conductivity, FCC metallic sublattice and variability of composition, in these NaCI-type carbides suggest a metal crystal in which carbon is interstitially dissolved; but in for example TiC there is almost no bonding between the metal atoms, virtually all the valence electrons having gone instead into carbon-metal bonding orbitals.

In recent years, following the development of density functional theory, there have been many calculations of the electronic band structures of these carbides, which have demonstrated the major role played by covalent bonds formed between the p states of the carbon atoms and the valence d states of the metal ones. Inevitably, the computational complexity of such calculations has largely limited their application to the N aCI-type structure and stoichiometric composition. To extend the theory to arbitrarily varied compositions and other structures a simplified model is thus needed.

My aim in this work, which is based on papers I have published recently in Materials Science and Technology, has been to seek an essentially qualitative understanding of all such carbides. The method is semi-empirical in that the parameters of its primary equations have been chosen from observed values. In this way it has been possible to set up equations for the cohesive energy which are consistent with the qualitative features of the band theory and yet are flexible enough to be applied, with second-order corrections, to a wide variety of compositions and structures.

Edited by: Alan Cottrell