Enhancing Effect of Iron Chlorides on the Anatase-Rutile Transition in Titanium Dioxide

The effect of solid Fe₂O₃ and gaseous iron chlorides on the anatase-rutile phase transition was investigated in the temperature range of 750°-950°C via X-ray diffractometry and scanning electron microscopy. In both cases, iron diffusion in the anatase lattice decreased the transition temperature and increased the anatase-rutile transformation rate, in comparison with that in TiO₂ fired in air. The enhancement effect of iron on the anatase-rutile transition is understood on the basis of oxygen-vacancy formation, which favors rutile nucleation. Solid-state iron diffusion between the contact points of TiO₂ and Fe₂O₃ particles and vapor mass transport through gaseous chlorides were the primary mechanisms of iron mass transport to the TiO₂ surface in the presence of both Fe₂O₃ and gaseous iron chlorides, respectively. The transformation rate at a given reaction temperature increased in the following order of reaction conditions: pure TiO₂ in air, TiO₂ in the presence of Fe₂O₃ in air, TiO₂ in the presence of Fe₂O₃ in chlorine, and TiO₂ in the presence of gaseous iron chlorides.