Changing the physical and chemical properties of titanium oxynitrides TiN1-xOx by changing the composition

The stability and structural properties of titanium oxynitrides, TiN1-xOx, of different compositions are theoretically analyzed by means of first-principles periodic density-functional calculations. We show that at x=0.55-0.6 there is a change in the preferred structure from that of NaCl type to the α-TiO arrangement. For the NaCl-type structure the cell volume increases with x while it decreases with x for the α-TiO structure. The bulk moduli are always much larger for NaCl-type structures than for α-TiO and they decrease as the amount of O increases, moving from 280 GPa for TiN to 226 GPa for TiO (NaCl-type structure) or 197 GPa for α-TiO. Changes in the electronic structure with the composition are also considered. In general we found that in the two types of structure (NaCl and α-TiO), both the band gap and the ionic character increase with the O concentration.