In situ high temperature EPR measurements of the growth of the signal of substitutional Fe(III) ions were 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 were studied. The first preparation, designated TiO2(SO4), was made from precipitated TiO2. It was calcined at about 850C and cooled slowly to room temperature. The second preparation, designated TiO2(Cl), was from the gas phase oxidation of TiCl4 at above 1200C. 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 up to 730C 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) versus time0.5 plots permitted estimates of the activation energies for the diffusion to be made. Values of 110kJ/mol for TiO2(SO4) and 50kJ/mol for TiO2(Cl) were determined. The much lower value for the TiO2(Cl) was attributed to the presence of metastable defects which, because of the rapid cooling, persist in this rutile. This interpretation was supported by an observed increase in activation energies on heating the rapidly quenched TiO2(Cl) prior to the diffusion experiment. Pre-annealing at 700C to reduce the concentration of defects, increased the activation energy for diffusion in TiO2(Cl) to 90kJ/mol. The activation energy for diffusion of Fe was significantly lower than that for Cr (150kJ/mol).

An EPR Study of Diffusion of Iron into Rutile. Egerton, T.A., Harris, E., Lawson, E.J., Mile, B., Rowlands, C.C.: Physical Chemistry Chemical Physics, 2001, 3[3], 497-504