Samples of Fe-doped rutile were studied using electron spin resonance methods. It was concluded that the Fe existed in at least 3 different states. Both substitutional Fe3+, and Fe3+ in rhombohedral symmetry, were distributed throughout the crystal. A third form could not be detected by using electron spin resonance methods, and existed only in a surface region which constituted less than 5% of the whole crystal. This region also contained other defect centers; even in samples which had not been doped with Fe.

The Distribution of Iron Centres in Fe-Doped Rutile Powders. Thorp, J.S., Eggleston, H.S., Egerton, T.A., Pearman, A.J.: Journal of Materials Science Letters, 1986, 5[1], 54-6