TiNF and Related Analogues of TiO2 : 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 Zn2 NF. In view of the important properties of TiO2 , we have attempted to prepare TiNF by employing an entirely new procedure involving the reaction of TiN with TiF4 . 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 TiO2 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 TiO2 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 TiO2 , and more interestingly N,F-codoped Ti3 O5 , 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-TiO2 and the TiNF phases. This has enabled us to understand the effects of N,F substitution in TiO2 in terms of the crystal structure, electronic structure and optical properties.