TiNF and Related Analogues of TiO ₂: A Combined Experimental and Theoretical Study

Aliovalent anion substitution in inorganic materials brings about marked changes in properties, as exemplified by N,F-codoped metal oxides. Recently, complete substitution of oxygen in ZnO by N and F was carried out to generate Zn ₂ NF. In view of the important properties of TiO ₂ , we have attempted to prepare TiNF by employing an entirely new procedure involving the reaction of TiN with TiF ₄ . While the reaction at low temperature (450 °C) yields TiNF in the anatase phase, reaction at a higher temperature (600 °C) yields TiNF in the rutile phase. This is interesting since the anatase phase of TiO ₂ also transforms to the rutile phase on heating. The lattice parameters of TiNF are close to those of the parent oxide. Partial substitution of oxygen in TiO ₂ by N and F reduces the band gap, but complete substitution increases the value comparable to that of the oxide. We have examined properties of N,F-codoped TiO ₂ , and more interestingly N,F-codoped Ti ₃ O ₅ , both with lower band gaps than the parent oxides. A detailed first-principles calculations has been carried out on structural and electronic properties of N,F−TiO ₂ and the TiNF phases. This has enabled us to understand the effects of N,F substitution in TiO ₂ in terms of the crystal structure, electronic structure and optical properties.