Electron spin resonance studies of impurity ions in rutile
Electron spin resonance techniques were used to study the photo-electronic behaviour of defect centres in rutile, using both single crystal and powdered samples. Single crystals of rutile were grown by a plasma torch method. Analysis of the boules by several techniques established that they contained, in general, fewer impurities than crystals grown by the conventional Verneuil method. Some boules were deliberately doped during growth with specific impurities. Control of the oxygen content of the plasma enabled the growth of some boules in a completely stoichiometric state, requiring no subsequent oxidation. Some crystals were doped after growth with various transition metal ions, using either evaporation and diffusion or vacuum capsule diffusion techniques. E.S.R. analysis indicated the presence of several impurity ions, notably iron and nickel, in most samples. Low temperature ultra-violet irradiation caused significant changes to the intensities of most spectra, together with the appearance of several new spectra. Isochronal annealing then revealed the temperatures at which thermally stimulated charge transfers occurred, as reflected in changes to e.s.r. spectra. At least twelve different trapping centres were detected, with thermal ionization temperatures between 30 and 400K. Where possible, the ionization energies were determined by measuring variations in the rate of charge transfer with temperature and many of the values were in good agreement with those reported using other techniques. In most cases it was also possible to determine the polarity of the traps, by studying their interaction with other centres. It is thought that many of the shallow electron traps consist of complexes of intrinsic defects, such as interstitial ions or vacancies. with nearby Aluminium ions. Iron and chromium appear to form hole trapping centres with energy levels near the valence band. Nickel, Manganese and Copper are recombination centres with levels near the centre of the band gap. The same e.s.r. methods were applied to samples of rutile pigments and a computer simulation technique was used to aid analysis and interpretation of powder spectra. All the pigment samples exhibited the same u.v.-activated e.s.r. spectrum. Isochronal annealing suggested that it represented a trapped hole and also inferred the existence of several shallow electron traps.