An EPR study of diffusion of chromium into rutile

Abstract

In situ high temperature EPR measurements of the growth of the signal of substitutional Cr(III) ions have been used to study the diffusion of Cr 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 TiO₂SO₄)], was made from precipitated TiO₂. It was calcined at ca. 1150 K and cooled slowly to ∽300 K. The second preparation, [designated TiO₂(Cl)], was from the gas phase oxidation of TiCl₄ at ca. 1500 K. The resulting titanium dioxide was then rapidly quenched to ∽300 K. The surfaces of both samples were impregnated with 300 ppm 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 temperatures up to 1000 K by using a high temperature EPR cavity. For both TiO₂(SO₄) and TiO₂(Cl) the g = 5.04 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. the time^1/2 allows estimates of the activation energies for diffusion to be made. Values of 150 ± 20 kJ mol⁻¹ for TiO₂(SO₄) and 65 ± 20 kJ mol⁻¹ for TiO₂(Cl) are determined. The much lower activation energy for the TiO₂(Cl) is attributed to the presence of metastable defects, possibly oxygen vacancies, which, because of the rapid cooling from 1500 K, 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 120 ± 30 kJ mol⁻¹.