First-principles calculations of vacancy effects on structural and electronic properties of TiC_x and TiN_x

First-principles calculations have been used to study the effect of vacancies on the structural and electronic properties in substoichiometric TiC_x and TiN_x. The effect of vacancies on equilibrium volumes, bulk moduli, electronic band structures and density of states of the substoichiometric phases was studied using a full-potential linear augmented plane-wave method. A model structure of eight-atom supercells with ordered vacancies within the carbon and nitrogen sublattices is used.

We find that the lattice parameters of the studied stoichiometries in both TiC_x and TiN_x are smaller than that of ideal stoichiometric TiC and TiN. Our results for the variation of the lattice parameters and the bulk moduli for TiC_x are found to be in good agreement with experiment. The variation of the energy gaps with the atomic concentration ratio shows that these compounds present the same trends. Results for TiC_x are compared to a recent full-potential calculation with relaxed 16-atom supercells.