Electronic Structure of Stoichiometric and Non-Stoichiometric Titanium Carbides and Nitrides

Abstract

ABSTRACT. In the present article a brief overview is given on the electronic structure and the nature of the chemical bonds in stoichiometric and substoichiometric titanium carbides and nitrides crystallizing in the sodium chloride structure. On the basis of APW band structure calculations it is shown that metallic, ionic, and covalent bonding occurs in the stoichiometric compounds. Three main types of covalent bonds can be distinguished: X 2p -Ti 3d 0 bonds, X 2p -Ti 3d n bonds, and Ti 3d -Ti 3d 0 bonds (X = C,N). The influence of vacancies at the non-metal sublattice sites on the electronic structure of the substoichiometric cubic phases TiX x was studied by two different approaches: (i) APW band structure calculations, assuming a hypothetical model structure with an ordered arrangement of vacancies, corresponding to the composition TiXO.75' (ii) KKR-CPA calculations, assuming a random distribution of vacancies and non-metal atoms over the non-metal sublattice sites. Both calculations lead to additional peaks ("vacancy peaks") in the density of states, induced by the vacancies. The changes in the covalent bonds, compared with the stoichiometric compounds, are discussed on the basis of local partial densities of states and electron densities. I • INTRODUCTIONRefractory transition metal carbides and nitrides crystallizing in the sodium chloride structure continue to be of considerable practical and theoretical interest. This class of substances exhibits, on the one hand, high melting points and ultrahardness -properties typical of covalent compounds -and, on the other hand, metallic properties, such as good electrical and thermal conductivity. Furthermore, these substances crystallize in the sodium chloride structure, which is generally found with ionic compounds. This unusual combination of properties has attracted an early and continuing interest in the electronic structure and nature of the chemical bonds in these substances.The electronic structure of the refractory transition metal

Band structure of substoichiometric titanium nitrides and carbonitrides: spectroscopical and theoretical investigations

“…The electronic structure of the corresponding stoichiometric and near-stoichiometric metal compounds has been studied by many authors for quite a long time (see, e.g., [3,4]) for the following two reasons. The first one is related to the synthesis methods which allow us to prepare these compounds very easily near the stoichiometry.…”

"Processamento, Caracterização Mecânica e Tribológica do Compósito Al2O3-NbC."

“…Dureza (GPa) Tenacidade à fratura(MPa m O carbeto de nióbio possui características semelhantes às do carbeto de titânio, são monocarbetos, isotípicos e cristalizam-se na estrutura cúbica de face centrada [44]. Cada átomo, metal ou carbono, é rodeado por seis vizinhos próximos da outra espécie atômica na configuração octaédrica, e a estrutura cristalina é apresentada na figura ll.7.…”