An EPR study of diffusion of iron into rutile

Abstract

In situ high temperature EPR measurements of the growth of the signal of substitutional Fe(III) ions have been used to study the diffusion of Fe in the rutile form of titanium dioxide. Two preparations, characteristic of the two main processes employed for the production of titanium dioxide have been studied. The first preparation, designated TiO₂(SO₄), was made from precipitated TiO₂. It was calcined at ca. 850°C and cooled slowly to room temperature. The second preparation, designated TiO₂(Cl), was from the gas phase oxidation of TiCl₄ at above 1200°C. The resulting TiO₂ was rapidly quenched to room temperature. The surfaces of both samples were impregnated with 0.030% Fe and the development of an EPR signal at g = 8.11, characteristic of Fe(III) substituting for titanium ions in the rutile lattice was monitored in situ at temperatures up to 730°C by using a high temperature EPR cavity. For both TiO₂(SO₄) and TiO₂(Cl) the g = 8.11 signal showed a parabolic dependence of intensity with time typical of many diffusion processes. The temperature dependence of the slope of the intensity (I) s. time^0.5 plots allows estimates of the activation energies for the diffusion to be made. Values of 110 ± 30 kJ mol⁻¹ for TiO₂(SO₄) and 50 ± 20 kJ mol⁻¹ for TiO₂(Cl) are determined. The much lower value for the TiO₂(Cl) is attributed to the presence of metastable defects which, because of the rapid cooling, persist in this rutile. This interpretation is supported by an observed increase in activation energies on heating the rapidly quenched TiO₂(Cl) prior to the diffusion experiment. Pre-annealing at 700°C to reduce the concentration of defects, increased the activation energy for diffusion in TiO₂(Cl) to 90 ± 30 kJ mol⁻¹. The activation energy for diffusion of Fe is significantly lower than that for Cr (150 kJ mol⁻¹). Reasons for this are discussed.