In situ high-temperature EPR measurements of the growth of the signal of substitutional Cr(III) ions were used to study the diffusion of Cr in rutile. Two preparations, characteristic of the two main processes employed for the production of titanium dioxide, were studied. The first preparation, [designated TiO2SO4)], was made from precipitated TiO2. It was calcined at about 1150K and cooled slowly to some 300K. The second preparation, [designated TiO2(Cl)], was from the gas phase oxidation of TiCl4 at about 1500K. The resulting titanium dioxide was then rapidly quenched to some 300K. The surfaces of both samples were impregnated with 300ppm of Cr and the development of an EPR signal at g = 5.04, characteristic of Cr(III) substituting for titanium ions in the rutile lattice, was monitored in situ at up to 1000K by using a high temperature EPR cavity. For both TiO2(SO4) and TiO2(Cl) the g = 5.04 signal exhibited a parabolic dependence of intensity with time, typical of many diffusion processes. The temperature dependence of the slope of the intensity (I) versus time1/2 permitted estimates of the activation energies for diffusion to be made. Values of 150kJ/mol for TiO2(SO4) and 65kJ/mol for TiO2(Cl) were determined. The much lower activation energy for the TiO2(Cl) was attributed to the presence of metastable defects, possibly oxygen vacancies, which, because of the rapid cooling from 1500K, 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 120kJ/mol.

An EPR Study of Diffusion of Chromium into Rutile. Egerton, T.A., Harris, E., Lawson, E.J., Mile, B., Rowlands, C.C.: Physical Chemistry Chemical Physics, 2000, 2[14], 3275-81