Apart from lattice vacancies, most real carbides, nitrides and materials produced from them contain another type of lattice defect: namely impurities, both interstitial and substitutional. These impurities may result in quite critical changes in properties of such phases, see the reviews of Samsonov, Upadkhaya and Neshpor (1974), Alyamovsky, Zainulin and Shveikin (1981) and Shveikin et al (1984). Moreover, doping of the compounds with atoms of other elements is one of the most direct and frequently used ways of changing the characteristics of initial phases and is extensively used in the synthesis of new materials with an improved set of technological parameters. For these reasons studies of the electronic properties of doped refractory phases and alloys have received a lot of attention in the last few years.

Many carbides and nitrides can form an unlimited number of solid solutions (SS), with physico-chemical properties which depend greatly on their composition. In some cases such concentration dependences exhibit nonmonotonic and rather complicated behaviour.

In this chapter we shall discuss the results of the theoretical modelling of electronic spectra, chemical bonding, charge distributions and properties of transition metal carbides and nitrides, doped by s and p impurities, as well as carbide- and nitride-based multicomponent solid solutions – carbonitrides, oxycarbides, etc.

Energy spectra of C, N, O impurities in Ti and Nb carbides and nitrides

The electronic structure calculations for C and O impurities in TiN were performed by Ivanovsky, Gubanov, Shveikin and Anisimov (1986a), using the semiempirical MO LCAO method.