CARBON VACANCIES IN TITANIUM CARBIDE

We present a simple tight-binding parametrization for TiC that is capa ble of giving qualitatively accurate results for a range of properties . The systematic procedure used to develop these parameters is describ ed, and the transferability of the model is tested. We use the model t o account for the existence of substoichiometric titanium carbide, and the variation of its lattice constant with vacancy concentration. We show that the unusual lattice constant trend arises from the fact that the lattice relaxation around a vacancy is highly localized. The phys ical origin of this localization is the strongly anisotropic strengthe ning of bonds around a vacancy, which follows from the quantum mechani cal nature of the bonding.