Electronic changes induced by vacancies on spectral and elastic propertiesof titanium carbides and nitrides

Substoichiometric carbides TiC0.49+ TiC0.78 and nitrides TiN0.45 TiN0.61 ha ve been synthesized by multiple energy ion implantation of C+ and N+ in pol ycrystalline titanium. Here, we review theoretical and experimental results concerning the electronic structure of these compounds with various degree s of vacancies. The valence band X-ray photoemission spectra (v-XPS) as wel l as core level XPS suggest important changes of the electronic structure i n comparison to the corresponding stoichiometric compounds. New structures appear near the Fermi level in the experimental v-XPS spectra in agreement with tight-binding-linear-muffin-tin-orbitals (TB-LMTO) calculations of the electronic densities of states (DOS). These structures, absent in stoichio metric compounds, are interpreted as resulting from Ti-Ti bond states betwe en neighbouring titanium atoms through vacancies induced by substoichiometr y along the whole range of vacancy concentration. Moreover, these bond modi fications explain the experimental changes of Young's modulus, observed by nanoindentation, in substoichiometric titanium carbides and nitrides. (C) 2 000 Elsevier Science B.V. All rights reserved.