An EPR study of diffusion of chromium into rutile

In situ high temperature EPR measurements of the growth of the signal of su bstitutional Cr(iii) ions have been used to study the diffusion of Cr in th e rutile form of titanium dioxide. Two preparations, characteristic of the two main processes employed for the production of titanium dioxide, have be en studied. The first preparation, [designated TiO2SO4)], was made from pre cipitated TiO2. It was calcined at ca. 1150 K and cooled slowly to similar to 300 K. The second preparation, [designated TiO2(Cl)], was from the gas p hase oxidation of TiCl4 at ca. 1500 K. The resulting titanium dioxide was t hen rapidly quenched to similar 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 la ttice, was monitored in situ at temperatures up to 1000 K by using a high t emperature EPR cavity. For both TiO2(SO4) and TiO2(Cl) the g=5.04 signal sh owed a parabolic dependence of intensity with time typical of many diffusio n processes. The temperature dependence of the slope of the intensity (I) v 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 t he TiO2(Cl) is attributed to the presence of metastable defects, possibly o xygen vacancies, which, because of the rapid cooling from 1500 K, persist i n this rutile. This interpretation is supported by an observed increase in activation energies on heating the rapidly quenched TiO2(Cl) prior to the d iffusion experiment. Pre-annealing at 700 degrees C, to reduce the concentr ation of defects, increased the activation energy for diffusion in TiO2(Cl) to 120 +/- 30 kJ mol(-1).